Lineament, Linear, Lineation: Some Proposed New Standards for Old Terms

Lineament, linear, lineation: Some proposed new standards for old terms

D. W. O'LEARY J. D. FRIEDMAN [ U.S. Geological Survey, Federal Center, Denver, Colorado 80225 H. A. POHN

ABSTRACT ORIGIN OF TERMS

The words "lineament," "linear," and "lineation" have become Lineament increasingly popular since the advent of spacecraft and high- altitude aircraft images. With the increased usage has come a con- The word "lineament" was introduced by Hobbs (1904) to comitant relaxation in the definitions of these terms, until the liter- characterize the spatial relationships of landscape features that in- ature now is overwhelmed with conflicting and equivocal mean- cluded (p. 485) "(1) crests of ridges or boundaries of elevated areas, ings. The need for clarification of these terms is clear. We propose a (2) the drainage lines, (3) coast lines, and (4) boundary lines of return to more fundamental conceptual definitions, based on origi- formations, of petrographic rock types, or of lines of outcrops." In nal usage. We define the word "lineament" in an essentially a later publication (1912), he added to these features ravines or val- geomorphological sense, on the basis of the usage introduced by leys and visible lines of fracture or zones of fault breccia. According Hobbs: A lineament is a mappable, simple or composite linear fea- to Hobbs, any number of these features may be joined end to end to ture of a surface, whose parts are aligned in a rectilinear or slightly constitute a lineament. He pointed out (1904, p. 485, footnote) curvilinear relationship and which differs distinctly from the pat- that lineaments are not equivalent to the "tektonische linien" of terns of adjacent features and presumably reflects a subsurface German writers, "as these are mostly regarded as lines of displace- phenomenon. The word "linear" is restricted to its original adjecti- ment." Hobbs felt that by attaching this meaning the term would val sense to avoid the increasingly popular but grammatically and be too restrictive. However, in 1912 (p. 227) he added a strong tec- conceptually incorrect nominative use. The word "lineation" is re- tonic implication to his definition by stating: "Many lineaments are stored to its fundamental petrographic meaning: lineation is the identical with seismotectonic lines and they therefore afford a one-dimensional structural alignment of internal components of a means of to some extent determining in advance the lines of rock, is imposed by external agents, and cannot be depicted as an greatest danger from earthquake shock." His actual definitions of individual feature on a map. In addition, we suggest usages of lineament are broad and metaphorical: "... significant lines of "line" and "alignment" to refer to nongeologic features and (or) landscape which reveal the hidden architecture of the rock base- questionable features that do not fit proposed criteria and where ment are described as lineaments" (1912, p. 227); "the term as here definitional restrictions or implications may be a problem. used is nothing more than a generally rectilinear earth feature" (1904, p. 485); ". . . they are the character lines of the earth's INTRODUCTION physiognomy" (1912, p. 227). However, his parenthetical notes give a better idea of what he had in mind. He pointed out (1904) Lineament, linear, lineation: what do these words mean and how that the linear aspect of lineaments is scale related: ". . . lineaments and why are they used? This question is pertinent in the face of a which may appear rectilinear on maps, may be so only in propor- growing number of studies in remote sensing, where much is made tion as the scale of the map is small" (p. 486). Lineaments "must be of the significance of linear features but where the drawbacks of conceived to outline in the majority of instances a complex but conflicting terminology have become increasingly apparent. There comparatively narrow zone of displacements, in which other direc- is often little agreement as to what linear features are because tions than that given by the general trend are included" (p. 486). criteria for recognition are not clearly established and because He also pointed out (1912, p. 227) the composite nature of linea- definitions are poor or inadequate. ments: "It is important to emphasize the essentially composite ex- The present state of confusion is underscored by the American pression of the lineament, but in every case it is some surface ex- Geological Institute's Glossary of Geology (Gary and others, pression of a buried feature." 1972, p. 408), where lineament is defined in three contexts: lunar, Following the reasonably clear and explicit definitions of Hobbs, photogeologic, and tectonic. The divergence of meaning implicit in a host of more or less divergent definitions have been proposed up the three separate entries, in addition to the nominative use of to the present day. These are listed chronologically in Table 1. linear as a synonym for tectonic lineament, illustrates the confused status of these terms and the lack of unequivocal definitions. These Linear terms need to be reappraised, not only to serve new insights and new applications based on remote sensor images, but also to The nominative use of the word "linear" has a relatively short achieve consistency between descriptions based on images and history. Sonder (1938) used the term to refer to individual features photos and descriptions based on field data. Our aim in this paper which Hobbs called lineaments; he proposed also that lineament be is to analyze the use of these terms and to propose conceptual used in a general, regional sense (p. 223, 225): "The lineament of a definitions and objective criteria for them. region then denotes a definite direction which is contained in the

Geological Society of America Bulletin, v. 87, p. 1463-1469, October 1976, Doc. no. 61011.

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tectonics, the jointing, and the relief." Gross (1951) elaborated this (p. 77): "The broken and sheared rock of a fault generally erodes concept and eliminated the term lineament altogether. He used the more rapidly than the solid rocks adjoining, and as a result faults word lineation to implicitly refer to the general trend of "linears," often occur in elongated topographic depressions called linears." as Sonder had used the word lineament: "The topographic lineation in any given direction may have any one of a number of causes Lineation and ... in general, it is folly to draw structural conclusions from a study of physiography alone" (p. 87). However, he did draw such Anderson (1915) first used the term "lineation," which he conclusions, and in doing so he seemed to confuse cause and effect defined (p. 22) as "a sort of striation visible on practically any

TABLE 1. REPRESENTATIVE GEOLOGIC USAGE OF TERMS "LINEAMENT" AND "LINEAR," 1904-1973

Reference Definition Comment

Hobbs (1904) A lineament is "nothing more than a generally rectilinear Term introduced to characterize spatial relationships of "1) earth feature" (p. 485) crests of ridges or boundaries of elevated areas, 2) drainage lines, 3) coast lines, 4) boundary lines of formations, or petrographic rock types, or lines of outcrops" (p. 485). Any of these may be joined end to end. Term is scale related: "lineaments . . . (are) rectilinear . . . only in pro- portion as the scale of the map is small" (p. 486). Line- aments tend to be obscure Hobbs (1912) Lineaments are "significant lines of landscape which re- Features include ravines, valleys, and visible lines of fracture veal the hidden architecture of the rock basement (p. or fault breccia zones but are in every case some surface 227); "the character lines of the earth's physiognomy" expression of a buried feature. Many are equivalent to (p. 227) "seismotectonic lines." They are composite features Sonder (1938) "The lineament of a region then denotes a definite direc- Used in a general, regional sense; individual features (com- tion which is contained in the tectonics, the jointing, pare Hobbs, 1904, 1912) are termed "linears" and the relief" (p. 223) Wilson (1941) Lineaments are "straight and gently curving lines" Emphasized structure: "Their association in definite pat- formed by "the great scarps and troughs which cross terns strongly suggests that they are members of con- all the Precambrian rocks" (p. 496) nected systems of faulting" (p. 496). Described as regional features better seen in air photos than on ground "A lineament is a straight linear surface feature that is "Lineaments are well shown on aerial photographs and . . . Kaiser (1950) at least many hundreds of feet and commonly many may consist of 1) linear topographic features, either miles long" (p. 1475) trenches or ridges; 2) linear vegetation patterns; 3) linear patterns of soil color or texture. Gaps and steam segments typically form parts of lineaments" (p. 1475) "The straight line or gently curved physiographic fea- "Linears" generally fault related; synonyms are "topo- Gross (1951) tures on the earth's surface are known as linears" (p. 79) graphic linear" and "lineal topographic feature." Used "lineal" as adjectival substitute for "linear." Used "lineation" to refer implicitly to general trend of linears (compare Sonder, 1938) 'In general terms a lineament is a rectilinear feature of Lineaments are usually "interrupted or discontinuous Kelly (1955) considerable extent on the surface of the earth. A tec- features" not readily mapped in the field and are "agree- tonic lineament . . . [is] either a general alignment of ments of boundary or line observed generally on regional structural features or a boundary between contrasting maps" (p. 58). Term is usually reserved for "transverse or structural features" (p. 58) oblique alignments of one sort or another" (p. 58), but included "in the broadest sense" (p. 58) are features such as the eastern Rocky Mountains Kupsch and Wild (1958) No specific definition Related lineaments to extended faults. Tonal lineaments "are believed to reflect differences in soil moisture result- ing perhaps from microrelief" (p. 129). Noted that "linear" is an etymologically undesirable term; suggested use of "linear trends, linear features, or lineaments" (p. 129) instead Lattman (1958) A lineament is "a natural linear feature consisting of topo- Lineaments are expressions of subsurface faults. Defined graphic (including straight stream segments), vegeta- "fracture trace" as a lineament "expressed continuously tion, or soil tonal alignments, visible primarily on for less than one mile" (p. 569), the expression of local aerial photographs or mosaics, and expressed con- jointing or small faults; included "only natural linear tinuously or discontinuously for many miles" (p. 569) features not obviously related to outcrop pattern of tilted beds, lineation and foliation, and stratigraphic contacts . . . included . . . are joints mapped on aerial photographs where bare rock is exposed" (p. 569)

Parkinson in Lueder (1959) "Any linear feature of the landscape which possesses an An implicit definition. "Bedrock linears . . . represent the abnormal degree of regularity . . . whether straight or surface traces of such geologic planar features as faults, gently curving, [is] generally believed to be the surface contacts, joints, and bedding planes" (p. 343). They also expression of some structural feature in the bedrock. represent "one of several structural features" (p. 343), Experience and careful judgement are required ... to including schistosity, gneissosity, or narrow dikes distinguish a diagnostic linear from random river stretches, hills, and similar results of random erosion" (P- 343)

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well-exposed divisional bedding-plane of the granulites. This linea- countably ignored this aspect of lineation and confined his interests tion is sometimes seen to be parallel over wide districts, even with entirely to the "megascopic to microscopic" scale. The original changes of strike." definition and subsequent ones are chronologically listed in This meaning was considerably expanded and cogently pre- Table 2. sented by Cloos (1946, p. 1): "Lineation is a descriptive and nongenetic term for any kind of linear structure within or on a Summary rock. It includes striae on slickensides, fold axes, flow lines, stretching, elongate pebbles or ooids, wrinkles, streaks, intersection of It is clear at the outset that "linear" and "lineament" stand in planes, linear parallelism of minerals or components, or any other very close relationship — to some minds they are synonymous — kind of linear structure of megascopic, microscopic, or regional whereas "lineation" is, with the exceptions discussed here, dis- dimensions." Apparently Cloos felt obligated to qualify the "re- tinctly apart in meaning and usage. Hobbs's definition of lineament gional dimensions" with which he had concluded his definition, is most valuable, not only because it draws on precedence but also and he stated: "On a large scale [sic] lineations can be seen on maps because it is well illustrated with descriptive example. It is mostly in the trends of folds or systems of folds like the Alps, the Ap- an implicit definition, but it describes the majority of characteristics palachians, or Andes." With this statement Cloos moved lineation that are commonly associated with lineaments. Lineaments (1) directly into the vocabulary of tectonics, regional and structural have geomorphic expression (in general, topographically negative), geology, and geologic remote sensing (compare with Gross, 1951). (2) are composite (either segmented or complex), (3) are charac- But apart from this statement on page 1 of his text, Cloos unac- terized by alignment in a single direction (which may or may not

TABLE 1. (Continued)

Reference Definition Comment

Hills (1963) "Megalineament is a term for large, somewhat vague, Megalineaments form definite patterns "over areas as large controversial lineaments of continental dimensions" as subcontinents or ocean basins" (p. 460) (compare (p. 460) Hobbs, 1904, 1912) Dennis (1967) Lineaments are "rectilinear or gently curved alignments Lineament "most commonly refers to regional features; of topographic features on a regional scale, generally 'linear' has a more local connotation and is used for lines judged to reflect crustal structure" (p. 102) of unsure origin on aerial photographs" (p. 103) Billings (1972) "In the broad sense of the word, a lineament is a line re- Lineaments presumably represent "the trace of a fracture or sulting from natural processes that may be observed or fracture systems on the surface. Most lineaments are inferred" (p. 208). "A lineament is expressed on the caused by steeply dipping faults or joint systems" surface of the earth as a relatively straight line" (p. (p. 419), but "the exact cause is unknown. The term may 419). "The lineament may be a long depression or a be used even if the cause is well established" (p. 419). long ridge" (p. 208) They may be easily confused with ridges and valleys re- sulting from the erosion of steeply dipping interbedded strata of varying hardness" (p. 208). Noted that the term "linear" is sometimes misused for "lineament"

Gay (1972) No specific definition Applied concept to aeromagnetic maps. Lineament is a "disruption in the contour pattern" (p. 5) manifested by two or more "elements": (1) termination of highs, (2) termination of lows, (3) change in contour gradient, (4) linear (straight) contour patterns. Elements may not be contiguous Brock (1972) "A lineament is defined briefly as a geological or topo- Types include "seismic," "coastal," and "island arc line- graphical alignment too precise to be fortuitous" aments." Scale is emphasized: "If it does not show up on (p. 187) a map it is not a lineament" (p. 188). They occur on all scales from air photos to outlines of continents (for example, the Andes, which are "rectilinear for 2,000 miles"; p. 187). Cited a tectonic relationship, but noted that no specific structure need be ascribed AGI Glossary of Geology Photogeologic (compare Allum, 1966, p. 31): "A line- May also be manifested by stream beds and alignments of (Gary and others, 1972) ament is any line on an aerial photograph, that is struc- vegetation, and so forth turally controlled . . . the term is widely applied to lines representing beds, lithologic horizons, mineral bandings, veins, faults, joints, unconformities, and rock boundaries" (p. 408) Tectonic: Lineaments are "straight or gently curved Have varied origins — for instance, "faults, aligned vol- lengthy features of the earth's surface, frequently ex- canoes, and zones of intense jointing . . . but the mean- pressed topographically as depressions or lines of de- ing of others is obscure . . . and their origins may be . . . pressions" (p. 408) purely accidental" (p. 408) Gay (1973) "Basement fracture lineament" (p. 101): a lineament Generally a direct manifestation of basement fractures (or observed directly on "basement" or its shallow cover faults) "Joint lineament" (p. 101): lineament caused by joint- Generally less than 1 to 2 km long; equivalent to Blanchet's ing not coincident with basement fractures (1957) "microfractures" and Lattman's (1958) "fracture traces." "Fracture trace lineament" (p. 101): compare Blanchet, Terminology and usage presumes an accurate knowledge of "macrofracture," Lattman, "photogeologic line- structural geology ament" Note: Key elements of definitions are in italics (ours).

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Reference Definition Comment

Anderson (1915) Lineation is "a sort of striation visible on practically Definition is collective and restrictive any well-exposed divisional bedding plane of the granulites. . . . This lineation is sometimes seen to be parallel over wide districts, even with changes of strike" (p. 22) Cloos (1946) "Lineation is a descriptive and nongenetic term for any "On a large scale [sic] lineations can be seen on maps in the kind of linear structure within or on a rock. It in- trends of folds or systems of folds like the Alps, the cludes striae on slickensides, fold axes, flow lines, Appalachians, or the Andes" (p. 1). Definition is com- stretching, elongate pebbles or ooids, wrinkles, prehensive, exhaustive, but equivocal streaks, intersections of planes, linear parallelism of minerals or components, or any other kind of linear structure of megascopic, microscopic, or regional dimensions" (p. 1) Fairbairn (1949) "Parallelism of linear elements shows as lineation" (p. 5) Definition is collective and related entirely to petrofabrics . Blanchet (1957) ". . . by the term fracture is meant the generally abun- Features have "marked rectilinear character" (p. 1748). dant, natural lineations discernible on aerial photo- Recognized two types: macrofractures and microfrac- graphs" (p. 1748) tures. Macrofractures are generally longer than 3.2 km (2 mi). Definition is genetic: "macrofractures . . . have been generated at the level of the basement, . . . the majority of microfractures have their origin above the half- way point in the sedimentary column" Compton (1962) "A lineation is a preferred (subparallel) orientation of Features include "crenulations in schist, intersections of one or more kinds of linear features" (p. 306) cleavages, sedimentary structures, etc." (p. 307). "Espe- cially useful are areas of folds whose closure can be observed in large outcrops or closely spaced groups of outcrops" (p. 307) Turner and Weiss (1963) 'All true lineations are penetrative discontinuities" ". . . the arrangement of grain boundaries in planar parallel (p. 31), that is, ". . . discontinuities in structure . . . orientation imparts to the whole aggregate a penetrative repeated at distances so small, compared with the planar structure . . . and ... a penetrative discontinuity scale of the whole crystal [or rock], that they can be ... is statistically present in every sample of the fabric" considered to pervade it uniformly and be present at (p. 22). "Penetrative surfaces" ("S-surfaces") include every point" (p. 21) foliation, bedding, and, at certain scales, joints. Features that define lineation, singly or in combination, include "1. Parallel elongate aggregates of individual minerals forming parallel streaks, trains, or bodies of cylindrical prismatic or spindle-shaped form.. . . Grains . . . may . . . be equant, tabular, or prismatic. To this general category also belong elongate rock fragments and ellipsoidal ooliths and microfossils whose inequidimensional outlines are due to deformation. 2. Linear preferred orientation of boundaries of prismatic or tabular crystals. 3. Axes of microcrenulations of a set of S-surfaces. ... 4. Intersection of two or more cozonal S-surfaces . . ." (p. 101) Dennis (1967) "Lineation includes all linear structures in rocks with- Compare Cloos (1946) out regard to origin. The term excludes purely super- ficial features, such as glacial striations" (p. 103) Billings (1972) "Lineation is the result of the parallelism of some direc- "Primary lineation" is caused in igneous rocks by flow of tional property in the rock, such as the long axes of magma or lava, and in sedimentary rocks by depositional hornblende crystals. Platy minerals or spherical grains agents. "Secondary lineation" includes features super- may be strung out in lines to produce a lineation" imposed on rocks by one or more phases of deformation (p. 328) following crystallization or deposition. Features include fold axes, "stretched" pebbles, crenulated foliation, mineral orientation, intersection of bedding and cleavage, slickensides, boudinage, rodding, and mullion structure. Lineation ranges in development from the fabric of nearly homogeneous rocks to the anisotropy of certain intensely deformed rocks AGI Glossary of Geology Sedimentary: Lineation is "any linear structure, of Definitions are equivocal. Meaning of "nongeneric" is un- (Gary and others, 1972) megascopic or microscopic nature, on or within a clear sedimentary rock" (p. 408) caused by primary deposition. Structural: Lineation is "a general nongeneric term for any linear structure in a rock of whatever scale" (p. 409)

Note: Key elements of definitions are in italics (ours).

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conform to regional trend), (4) are straight or slightly curved, (5) Lineaments may be either positive or negative features: geologi- are regional in extent, and (6) are scale related. Hobbs and other cally they may reflect the erosive resistance of a subsurface feature authors have agreed that lineaments are essentially geomorphic fea- or of material modified by such a feature; geophysically they may tures and that they are commonly or probably related to structural reflect a subsurface anomaly trend or disruption. Tonal features discontinuities (chiefly faults, shear zones, and joints) — an associ- may reflect a subsurface material or property discontinuity of some ation emphasized but not caused by erosion. sort. In any case, a lineament is exceptional in the sense that it The noun usage of the word "linear," in its most detailed exposi- reflects some sort of anomaly or discontinuity within the surround- tion, is a synonym for lineament. Dennis (1967) felt that it should ing terrain. refer to linear features of vague or uncertain nature, whereas Bil- The definition is general, and a few qualifying points are perti- lings (1972) emphatically labeled its use a catachresis. Tradition- nent. The word "lineament" is often used with a modifying term to ally, linear has been used as an adjective in geological writing (Balk, indicate its chief manifestation — for instance, "topographic line- 1937; Cloos, 1946); attempts to supplant it with lineal have not ament." This is perhaps useful for characterizing certain features, gained wide acceptance. but longer composite lineaments may not be described adequately Lineation is a structural-petrographic term, the sense of which in this way. It would be useful to retain modifiers such as "tonal," may be singular or collective. Cloos (1946) first presented those "topographic," or "magnetic" to refer specifically to particular characteristics that have been commonly accepted by later writers. kinds of expressions; the words "airphoto," "radar," and so on are Lineation is (1) nongenetic, and the term refers purely to geometri- not adequately descriptive modifiers and should be dropped, as cal features, (2) truly one dimensional — that is, a line or lines they refer to a kind of sensing or recording system and are not in- defined within a plane or by the intersection of two or more planes, trinsic to the character of a lineament. A characterization of inves- or by a variety of other kinds of features, and (3) large scale, rang- tigator confidence is commonly made by use of such words as ing from megascopic to microscopic. (Lineation may be consistent "pronounced" or "vague." Words of this sort are broadly subjec- on a regional scale, but it cannot be observed on a regional scale.) tive, so each investigator should carefully qualify the sense of these words if their use is necessary. PROPOSED DEFINITIONS OF LINEAR FEATURES Although a line, according to definition, need not be straight, lineament is defined as representing only those lines that are Lineament straight or slightly curved. Certain linear features may manifest all of the typical lineament characteristics but may be strongly curved. Lineament. L. linea = line + L. mentum = akin to; hence, akin Distinctly arcuate features might be called curvilineaments with no to or like a line. A lineament is a mappable, simple or composite other modification in terminology. linear feature of a surface, whose parts are aligned in a rectilinear or slightly curvilinear relationship and which differs distinctly from Linear the patterns of adjacent features and presumably reflects a subsurface phenomenon. Linear. L. linea = line + L. aris = of, or belonging to; hence, of Surface features may be physiographic (expressing relief) or or relating to a line; denoting the properties of a line. Linear is an tonal (expressing contrast). Physiographic expression consists of adjective that describes the linelike character of some object or ob- isolated or continuous landforms, the boundaries of morphologi- jects. cally distinct areas, or breaks (including stream courses) in a uni- The word "linear" should not be used as a noun substitute for, form terrain; "surface feature" also includes forms other than top- nor as an approximation of, lineament. Linear has been used con- ographic, such as those manifested by aeromagnetic or gravity po- sistently and has priority as an adjective in geologic terminology; tential field surfaces. Tonal expressions include discontinuities, the precedence for this usage is so well established that it is mislead- whether boundary lines or "stripes" (including those manifested by ing to introduce a different, specifically nominative meaning while vegetation). These discontinuities may be discerned either on the the adjectival usage is so widely accepted and necessary. The sense ground surface or on an image of the ground surface as, for exam- of the etymon is clear: "relating to a line." Thus, linear belongs to ple, a thermal image. the class of words that expresses the geometrical qualities of things. Physiographic or tonal surface features must be recognizably To identify that which has the quality of being linear as a linear is linear to qualify as lineaments. Lineaments may be simple (a single like identifying that which is hot as a hot or that which is green as a continuous feature or a series of a single kind of feature) or they green. It is an unwarranted generalization. To be consistent in the may be composite (a variety of contiguous or detached features). use of linear as a noun, we would have to countenance the nomina- The features, if separated, are aligned in a succession that is natural tive use of curvilinear and rectilinear, and we would likewise have (that is, noncultural), recognizable, and relatively closely spaced. planars, circulars, triangulars, and so on as real objects. This succession is manifested by features that either are obviously An object cannot be defined in terms of itself. Thus, while it is related genetically, or are closer together than are surrounding fea- informative to say that a lineament is linear, it is not worthwhile to tures, or are themselves elongated in a unique direction. A distinct say that a linear is linear. In the first case we say that an object is change in direction of alignment represents a different lineament. characterized by linearity; in the second case we say that an object At largest scale, lineaments may be identified with a single map is essentially itself — which provides us with no information. It is unit; at smallest scale, they are expressions of a landscape and may not good logic to identify subject and predicate with the same be continental in extent. Hence, actual length is relative to the scale word. If it is held that a "linear" is simply a kind of lineament, it is of observation and cannot be arbitrarily limited. The features that a lineament all the same, and linear at that. constitute a lineament at one scale may not constitute one at another. Scale should be specified if different scales are considered Lineation in a single study. We suggest that "mappable" and "map unit" be understood in the sense proposed by the American Commission on Lineation. L. lineatus = past part, of lineare = to make straight Stratigraphic Nomenclature (1961, p. 650): "Mappability at the + L. ion = result of an act or process; hence, made straight or con- surface is considered as delineation at scales of the order of formable to a line; a line. Lineation is the one-dimensional struc- 1:25,000." As manifestations of real features, lineaments have tural alignment of internal components of a rock, is imposed by ex- width, so at the scale of observation the ratio of length to width ternal agents, and cannot be depicted as an individual feature on a must clearly be in keeping with a linear aspect. map.

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Lineation is a petrofabric term that refers to either a single fea- implication in many aspects of geology and remote sensing. We ture or to a parallel relationship of many features throughout a suggest that the word "line" be used to describe informally any rock. The alignment is recognized prior to identification of the nongeologic linear feature of whatever scale and, provided the fea- components that manifest it. It may involve more than one kind of ture is truly "linear," to refer to questionable features that do not component, and hence a lineation may be defined by more than one fit proposed criteria. The word "alignment" may be used to de- kind of structure; the alignments may not coincide, and hence a scribe informally any line defined by an alignment of terrestrial fea- rock may have more than one lineation. The alignment may be se- tures (or even extraterrestrial ones), regardless of spacing or genetic quential, defining a single line, or it may be parallel, defining a association. group of lines. The components may be discrete, contiguous, or Thus, agricultural and land-use patterns, certain cultural fea- continuous. If discrete, the components may be equant or elongate; tures, and weather-caused surficial features might appear as lines if they are elongate, the alignment of axes expresses the lineation; if on an image, as might certain features of possible geologic they are equant, segregation into linear concentrations expresses significance. A line connecting more or less widely spaced features this. such as ore deposits, intrusions, or volcanoes would constitute an The relationship of the components that constitute the lineation alignment. The significance of an alignment would be purely in- is one dimensional; hence, for any given lineation, there is a unique terpretive, as the line connecting the features presumes a relation- orientation in which the lineation cannot be seen and a unique ship of some sort. orientation in which it is most obvious. The components that make up a lineation range in size from microscopic to megascopic and CONCLUSIONS may be grains or crystals, blebs within grains, clusters of grains, distortions, contours, discontinuities, inclusions, clasts, and so on. We have proposed a revised and more versatile definition of the In any case the components are distinct and integral structural word "lineament," meant to be applicable to the Earth sciences in parts of the rock, not simply surface characteristics. Surface effects general and to geologic remote sensing in particular and to be dis- such as striae or intersections of two joint planes are not properly tinctly different in meaning from the words "linear" and "linea- components of the rock. As such, they do not constitute lineations tion." Two principles are stressed: exclusion of overlapping terms in the above sense, although they have been defined elsewhere as and recognition of the distinction between what a feature is and lineations (Cloos, 1946; Billings, 1972), because, as Billings stated what causes it. We define a lineament as a mappable, simple or (p. 410), "the intersection of two planes is a line." By this definition composite linear feature of a surface, whose parts are aligned in a one might include as lineations the intersections of any tabular fea- rectilinear or slightly curvilinear relationship and which differs dis- tures or structural planes with any surface under observation (out- tinctly from the patterns of adjacent features and presumably crop, thin section, or the like). Hence, the edges of veins, fractures, reflects a subsurface phenomenon. The word linear is restricted to cleavages, or other surfaces would be lineations in the loose terms its adjectival meaning. We define lineation as the one-dimensional of Cloos's definition. This equivocal meaning comes about because structural alignment of internal components of a rock, which is im- Cloos's definition includes at least four distinct kinds of linear fea- posed by external agents and cannot be depicted as an individual tures: (1) true internal lines (for example, flow lines or apices of feature on a map. Our point of view is tailored to the needs of contours), (2) edges or intersections of internal planes (for exam- identification and classification, and our definitions are generic. ple, foliations), (3) lines on or defined by geometry of surfaces (for Thus, the actual meaning and significance of lineaments is a matter example, striae, intersection of cleavage), (4) regional structural far beyond the rationale of definitions. We feel that the recognition trends (for example, axes of foldbelts), as well as "any other kind of lineaments as real, observable geologic features is of fundamen- of linear structure of megascopic, microscopic, or regional dimen- tal value in its own right. As Brock (1972) pointed out (p. 187): "In sions" (Cloos, 1946, p. 1). ignoring lineaments until faulting has been proved, one is denying We favor restricting the definition to features integral to the oneself a vast potential source of information concerning funda- structure of a rock ("penetrative" in the sense of Turner and Weiss, mental crustal patterns which would remain grossly incomplete 1963) — that is, the first and second cases cited by Cloos. We feel without the help of lineaments." that features of free surfaces (such as striae or intersection of joints and [or] bedding) and regional structural features should not prop- ACKNOWLEDGMENTS erly be called lineations, as this leads to equivocal usage of the term, and these features can be adequately described by other struc- We thank Gordon Eaton for his thorough and trenchant review tural terms. of this paper and for his valuable contributions. We also thank The term "lineation" is quasi-genetic, as a stress origin is implied Robert Compton for his advice concerning the definition of linea- in the definition. The term is, in fact, valuable for its genetic conno- tion and Terry Offield, Mike Pease, John Peper, and Pat Barosh for tations, as it may refer to features imposed either by the deposi- their stimulating comments and suggestions. tional agent (primary) or by external forces following deposition or consolidation (secondary). Lineation may be symbolically represented on a map, but it can- REFERENCES CITED not be individually represented as can a formational contact. Thus, formational contacts are conventionally depicted by some kind of Allum, J.A.E., 1966, Photogeology and regional mapping: New York, Per- line, but at the same scale lineation can be shown only symboli- gamon Press, 107 p. cally. 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