Pnoc. \'1<:0£, .AssOC ..\'(l). .XXVIII. PLATE 1. Alluvium InlrlJsions CDrhonif.rou~ afld Trios Rhyolites (Jpptr Andesites Harroth Tuffs Wrengill Anduites Midtllt Tuffs !.Qwer Cold Andeaifu r~1I MoftledTuffs lofterharrowSond! THE WATFORD ENGRAVfNG Co l!~ Skit/dow 1 J~ 0 1 .~.4 .<C" ---~ Slat. S.lAIIlBS CONOUIT 5: HOLBORt-l.W.C -'''_111:::='''_1IIt:::=i' .._.?iI:::======:i. Jiit::======:I£__ ...T MAP OF THE COUNTRY BETWEEN WILTON AND BORROWDALE By J FRED K N. GREEI" --=.G.S. Jlnoc. G:F.OL .ASSOG.,VOl~.xxvrll. PLATE 2. Alluvium Intrusions Corboniferous and Trios Sltules 1coniSfon Rhyolite limestone . Grit Series lower Ande$ite.~ + Hut/ooRoo Skiddow SIC/Ie THE WATFOR:J ENGRAVINGC~LT.o Scalf!- ofNiles J 'n ~"~ "'UI...=~1=~;;-"",===>-=--~-·f 5. L...",r,s C.ONDUIT S~ ~:OLB()PI·I. w.c. I {>. , .'- ....j========it'.... ...{ l\'lAP OJ<' THE COUNTRY BETWl~]~N COCKERMOUTH --e'\..ND CARRICK F:f~LI,. Bv .J. FRED~ N. GREEN,F.G.S. ( NotR,. The. 1.Jount.wy 01' tlw Corboniferousi,8 fn.ken,froTrV lhb maps oft}w Ge.ologi.caL Survey) PROCEEDINGS OF THE GEOLOGISTS' ASSOCIATION. THE AGE OF THE CHIEF INTRUSIONS OF THE LAKE DISTRICT. BY]. F. N. GREEN, B.A., F.G.S. (Read January 6th, 1917.) PAG£~ I. THE PROBLEM [ II. HISTORICAL .•..• 2 III. THE SKIDDAW-BoRROWDALE JUNCTION. Unfaulted character. The Latterbarrow Sandstone. Supposed lagplanes. The purple breccia of Falcon Crag .• 5 IV. STRUCTURAL. I. The Eskdale Granite. The Devoke Water Thrust. 12 2. The Buttermere Granophyre . 1+ 3. The St. John's Microgranite . IS 4. The Carrick Fell Complex 17 V. STRATIGRAPHICAL. The Watch Hill Grits. Bearing of their composi- tion on the Carrick Fell problem. The Drygill Shales. Ordovician denudation Ie) VI. GENETIC. ••• 26 VII. THE QUARTZ-DOLERITES ••.• 27 VIII. CORRELATION OF THE BORROWDALES • 28 1. THE PROBLEM. CONDITION precedent to the useful discussion of the A evolution of igneous rocks, either within a limited area or more generally, is the determination of their succession by a study of their field relations. In the most remarkable igneous area in Britain, namely, the Lake District, the writer has endeavoured to find a clue to the sequence and structure among the volcanic beds by detailed mapping of selected portions. Naturally, the complication of intrusion and metamorphism has as far as possible been avoided; but in following up outcrops of various horizons, it was inevitable that observations should be made bearing on the relations of the larger intrusions. Some of these yielded results which wen.' important, though unexpected, and so this paper is the outcome of notes made originally with another object. PROC. GEOt. Assoc., VOL. XXVIII, PART I, [9I7.J :z ]. F. N. GREEN ON The chief intrusions among the older Lake District rocks are acid. Basic masses, usually quartz-dolerites, occur, it is true, of considerable size, as, for example, at Carrick Fell, Great Cockup, Sea,thwaite, Castle Head, and Haweswater. But the large outcrops are predominantly acid, except in the case of Carrick Fell, where about half the exposure is basic. It is not easy to draw the line between major and minor intrusions. The former may be taken to be those known as the Shap granite, the Eskdale granite, the Buttermere granophyre, the St. John's microgranite, the Skiddaw granite, and the Carrick Fell complex. The total exposed area of these is about 60 square miles. The Shap granite need not be further considered, as its penetration into the Silurian, with the occurrence of boulders in the basal beds of the Upper Palreozoic, enabled its age to be determined within narrow limits half a century ago. No sort of assurance exists for the others, and the most diverse opinions have been expressed. Four come in at the same horizon, the junction of the Skiddaw Slate with the overlying volcanics; and these four show similarities in a number of other points, notably the im• portance of soda in their chemical composition, the tendency to granophyric structure, and the form of the intrusion as a sill or depressed laccolith. The fifth, the granite of Skiddaw, though, owing to the wide metamorphism accompanying it, attracting more attention than the others, is much more obscure, being in contact only with the lower part of the Skiddaw Slate Series, and of indeterminable form. II. HISTORICAL. Sedgwick* had observed that the main granite masses were later than the Skiddaw Slates, and seems ;to have considered some of them earlier than the greater part of the volcanic series. He was quite definite on their relation to the main crust" movements, writing- I believe that the greatest elevations and conto-rtions of the slates took place after the eruption of the granite and syenite. Harkness,t in 1863, regarded the granites of Skiddaw and Caldbeck as part of the same mass. Nicholson,t in 11>69, was disposed to look upon the igneous masses in question as the roots of the ancient vents from which were derived the alternating ashes and traps. • .. Guide to the Lakes," (Letters to Wordsworth, t842.) t Quart, Jou.... Gear. Soc.. xix, p. 124. t II Notes on certain of the Intrusive Igneous Rocks of the Lake Distdct." Quart. Jou"", Gl<Jl. Soc., nv, p. 435. THE AGE OF THE CHIEF INTRUSIONS OF THE LAKE DISTRICT. 3 Ward* however came to the conclusion, chiefly from his examination of the microscopic cavities in the quartz, that all these intrusions must be associated with the Shap Granite, and were to be assigned to the time of the great crust-movements, that is, the close of the Silurian. In 1895 Mr. A. Harker, in the course of his classical study of the Carrick Fell complex,t said;- The rocks have been intruded at the junction of the Skiddaw Slates and the Eycott Hill lavas· since these two· ·~roups assumed their present mutual relations. There is reason to believe that these relations are not the natural ones, and that the absence here of certain other groups which should intervene is a result of the great post-Silurian crust· movements. If this be 50-, the intrusions mus.t be later than these crust-mo'vements. The abs,ence of cataclas,tic structures, or any phenomena attributable to shearing in the intrusive masses, is, so far as it goes, a confirmation of this view. The faults, which have displaced the intrusive rocks. are to be referred to the later (post-Carboniferous) disturbances. In the discussiont he stated that the Skiddaw granite and greisen belonged to the epoch of the post-Silurian movements. In 1900 Dr. Marr§ held that the Skiddaw granite had been forced along the thrust-plane, and the St. John's Vale and Buttermere intrusions along the lag-plane, which, on his hypo• thesis, were required to account for the structure of the district. He suspected that the Eskdale granite was also intruded along a lag-plane, and suggested that the Skiddaw granite was De• vonian, but that the Carrick Fell rocks, regarded as lying in the same thrust as the Skiddaw granite, were more modern. In 1902 Mr. Harkerl! divided the intrusions into an older and a younger suite. In the former he included the Buttermere granophyre, the St. John's microgranite, and the Great Cockup picrite; in the latter the granites of Shap, Skiddaw, and Esk• dale, and the Carrick Fell complex. Dolerites belonged to both sets. In 1906 Mr. R. H. RastallJ~ in the course of his description ()f the Buttermere and Ennerdale Granophyre, wrote;- The on,ty ·evidence bearing 0'11 the poinrt so far obtainedi, is the absence of cleavage on the north s,ide of the Buttermere laccolite, which seems. to show that the Skiddaw Slates on this side were pro• tected from the influence of the great thrusts coming from the south· east, which so ,strongly cleaved them in other parts of the district. The intrusion was therefore previous to the great earth-movements- of the later part of the Caledonian series. Whether it accompanied the Ordovician movements of the same series remains to be proved. • "On the Granitic, Granitoid and Associated Metamorphic Rocks of the Lake District." Quart. Journ. Geol. Soc., Parts I-II, xxxi (1873), p. 368; Parts III·V, xxxii (1876), p. I. t Quart. Journ. Geol. Soc., Ii (1893). p. 126. t Ihid., p. 148. § II Notes on the Geology of the English Lake District." Proc. Geol. Assoc., xvi, p·449· II Proc. Yorks Geol. Soc .•. xiv, p. 489. , Quart.]ourn. GeoJ. Soc,. IxU, p. 26g. 4 ]. F. N. GREEN ON In the same year Dr. Marr, in his Anniversary Address to the Geological Society,* said:- The Buttermere granophyre and some sills and dykes, chiefly occurring in and around the central fells, are probably of Ordovician age; and the Eskdale granite may be of that age, though possibly newer. But he suggcstedt a Tertiary age for the Carrick Fell complex, comparing it with the Tertiary igneous rocks of Scotland. In a paper read in r908 and published in the following year, Dr. A. R. Dwerryhouset supported on the whole the current view that the Eskdale granite belonged to the Devonian period. In the discussion on this paper, however, the present writer expressed§ the opinion, as a result of his examination of the southern edge of the mass, that it was older than the main crust-movements. In 1910 Mr. RastaU,11 in a paper on the Skiddaw Granite and its Metamorphism, stated- it is clear that the Skiddaw Slates had undergone very extensive dynamic metamorphism previous to the intrusion of the granite, since the deavaJgle', £()llia.tion, and fol(iill1gare still clearly visible, alithowgh the divisional p1aIlJes hJave be<~n.