BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA V o l . 11, PP. 461-474, PLS. 43-49 JUNE 30, 1900 THOMSONITE, MESOLITE, AND CHABAZITE PROM GOLDEN, COLORADO BY HORACE B. PATTON (jRead before the Society, December SO, 1899) CONTENTS Page Work of previous investigators.................................................................................. 461 Place and mode of occurrence................................ ................................................... 462 Thomsonite..................................................................................................................... 463 General description.............................................................................................. 463 Type 1....................................................................................................................... 464 Type I I ................................................................................................................... 466 Type 11a................................................................................................................... 466 Type III................................................................................................................ 467 Type Ilia .................................................................................................................. 467 Mesolite...........................................................................................................................468 Chemical analyses.......................................................................................................... 469 Chabazite...................................................................................................................... 470 Other zeoli tes.................................................................................................................. 472 Analcite..................................................... ............................................................. 472 Apophyllite.............................................................................................................. 472 Stilbite and laumontite.............................................................: .......................... 472 Calcite and aragonite............................................................................................. 473 Order of deposition..................................................................................................... 473 Summary......................................................................................................................... 473 W o r k o f P r e v io u s I nvestigators The zeolites of North and South Table mountains at Golden, Colorado, have already become known to mineralogists mainly through the ex­ cellent descriptions of these minerals and of their occurrence by Cross and Hillebrand in Bulletin 20 of the United States Gelogical Survey.* In this bulletin the above named authors t have given a brief description of ♦Contributions to the Mineralogy of the Rocky Mountains, Bull. no. 20, U. S. Geol. Survey, t A briefer, preliminary paper by the same authors was published in A.m. Jour. Sci., 3d ser., vol. xxiii, 1882, p. 452, and 3d ser., vol. xxiv, 1882, p. 129. LXVI—Bum.. Geoi.. Soc. Am., Vol. 11, 1899 (461) 462 H. B. PATTON— THOMSONITE, MESOLITE, AND CHABAZITE the Table mountains, with their basaltic caps and a much more detailed description of the various zeolitic minerals contained in the amygdaloidal cavities. They also discuss the results of the investigations of Professor Carl Klein on the optical anomalies of analcite and apophyllite from this locality* During the past two years the Colorado State School of Mines, located at Golden, has been conducting quarrying operations for the purpose of securing for its mineral cabinet specimens of these Table Mountain zeolites. A new locality opened up on the east faGe of North Table mountain proved to be very prolific of these zeolites and produced many specimens of ex­ treme beauty. In most respects the zeolites here developed correspond closely with the occurrences described in the above mentioned paper by Cross and Hillebrand, and it is hardly worth while to duplicate what they have already written on the subject. The minerals thomsonite and mesolite, however, not only show extraordinary beauty, but occur in a great variety of forms; habits, and associations, and in the case of the former also present features that do not entirely accord with the descriptions of Cross and Hillebrand. P l a c e a n d M o d e o f O c c u r r e n c e As the description of these zeolites and of the Table mountains in which they occur are so readily accessible to all, a very brief reference to the surroundings will suffice for our present purpose. At Golden are two socalled table mountains, designated North Table mountain and South Table mountain. Geologically they are but one mountain, con­ sisting of soft, nearly horizontal beds of Middle Tertiary age, capped with a thick lava sheet and cut in two by Clear creek. The soft bedded rocks that form the base of the mountain are almost entirely composed of fine andesite ash beds belonging to the Denver Tertiary.f The lava cap consists of two flows of feldspar-basalt that together reach a thick­ ness of about 100 feet at the place where the minerals under discussion were obtained. The second flow followed so closely upon the first that no erosion of the first sheet occurred before it was covered by the second sheet. The two flows are of about equal thickness and form a nearly vertical cliff of 100 feet, in the center of which is a horizontal band of very scoriaceous basalt that belongs mainly to the top of the lower flow. This porous band is some 15 feet thick and contains caCvities of all sizes up to 6 or 8 feet. The large cavities are drawn out flat in the direction ♦Neues Jahrbueh fur Mineralogie, etc., vol. i, 1884, p. 250. fSee Whitman Cross: Geology of the Denver basin, Monograph xxvii, U. S. Geol. Survey, p. 155. PLACE AND MODE OF OCCURRENCE 4 6 3 of flow; the smaller ones may be flattened oval in shape. Even in the weather-beaten face of the cliff the presence of white zeolitic minerals filling the cavities is very noticeable. At a depth of 2 or 3 feet they are usually quite fresh. Cross and Hillebrand divide the zeolites filling the amygdaloidal cav­ ities in the basalt of North and South Table mountains into two groups, based upon their method of occurrence. The minerals of the first group are laumontite and stilbite, and are to be found only on the floor of the cavities. On the floor of many of the cavities, especially of the larger ones, there is to be seen a very peculiar bedded deposit of yellowish or reddish yellow color, which closely resembles a friable sandstone, but which is shown to be composed of mixtures of these two minerals. Included in the minerals of the first group are also to be mentioned occasional minute, reddish spherules of thomsonite. These latter have not been observed by the writer in the newly opened locality ; but with this exception the minerals of the first group are quite in accordance with the descriptions given by Cross and Hillebrand and may be passed over without further comment. The second group is made to include all those that are not confined to the floor of the cavities. They occur on the roof and sides, as well as on the floor composed of zeolites of the first group, or they completely line cavities that do not contain these bedded floors. The minerals of this second group are thomsonite, stilbite, chabazite, analcite, apophyllite, and mesolite, to which maybe added calciteand aragonite, as both these minerals occur associated with the zeolites of the second group. T h o m s o n it e GENERAL DESCRIPTION It is universally recognized that specimens of any given species com­ ing from the same locality usually have the same habit and general appearance. This similarity affects crystal form and habit, color, luster, size, and association, and is so marked that mineralogists do not hesitate to identify localities by such peculiarities, such identification of localities usually being entirely justified by the facts. Exceptions to this rule will doubtless suggest themselves to the reader, but it is doubtful if a more striking one can be found than is shown by the mineral thomsonite in the particular locality under discussion. It would not be difficult to select four or more specimens of this mineral coming from immediately adjacent cavities so markedly different in habit and general appearance as to suggest their occurring in widely different localities. In fact, one 464 H. B. PATTON— THOMSONITE, MESOLITE, AND CHABAZITE may be justified in stating the contrast in still stronger terms, inasmuch as it is difficult to realize that the several specimens really represent the same mineral, irrespective of their common origin. This great diversity is undoubtedly due to the fact that the several varieties have been formed under varying conditions. It is, indeed, possible and easy to recognize several generations of thomsonite, each generation having its own habit. As a matter of fact, two such generations were recognized and described by Cross and Hillebrand. Their description of the first generation is very exact, and specimens of this type may easily be identified from such description. It is not so easy, however, to identify their second genera­ tion with any individuals of the later ones described in the following pages. Without the aid of photographs