D.Sc THROUGHOUT Western Renfrewshire the Rocks Exposed
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Downloaded from http://trngl.lyellcollection.org/ at Cornell University Library on July 13, 2015 Vol. xvi.] LEITCH—RQCKS OF WEST RENFREWSHIRE—SCOTT. 275 XIV.—NOTES ON THE INTRUSIVE ROCKS OF WEST RENFREWSHIRE. By P. A. LEITCH, A.M.Inst.C.E., and A. SCOTT, M.A., D.Sc [Read 8th March, 1917.] [Issued separately, 30th December, 1917.] INTRODUCTION. THROUGHOUT western Renfrewshire the rocks exposed are prac tically all of Old Red Sandstone or Carboniferous age. Sedi ments of Calciferous Sandstone age outcrop along the shore from Port-Glasgow to Wemyss Bay, and also to the south-west of Loch Thorn, where they are underlain by Old Red Sandstone. A syncline with an approximately east and west axis occurs south of Greenock, but further to the south-west, Old Red Sandstone sediments are exposed in the middle of an anticline, the axis of which trends north-north-east and south-south-west. The remainder of the county is composed of the lavas and tuffs of Calciferous Sandstone age, together with a number of isolated intrusions and the volcanic vents of the Misty Law district. As the last named should more properly be con sidered in connection with the vents of north Ayrshire (Wilson, 1916) we do not propose to treat them in detail here. The intrusions may be divided into three groups. I. Basaltic Rocks of Calciferous Sandstone Age.—These com prise the sills of Lunderston Bay and Gryfe Castle, and a number of dykes in the Gourock and Kilmacolm district. The field evidence indicates that these are later than the sediments, while their petrological character assigns them to the Calci ferous Sandstone volcanic episode. II. Felsitici Rocks of Calciferous Sandstone Age.—These in clude the sill at Craigmushet, a number of intrusions to the south-west of Loch Thorn and a sill (?) near Kilmacolm. With the exception of the last, these are all intrusive into the sedi ments, but there is little doubt that they do not represent the earliest phase of Carboniferous activity. They are probably of the same age as the acid rocks of the Cumbraes (see pre- VOL. XVI., PT. II. T Downloaded from http://trngl.lyellcollection.org/ at Cornell University Library on July 13, 2015 276 GEOLOGICAL SOCIETY OP GLASGOW. [Trans. ceding paper), and hence almost the latest of the igneous rocks of Calciferous Sandstone age. III. East-West Dykes of Permo-Carboniferous (?) Age.—The chief intrusion of this group is a large dyke near Kilmacolm. PETROGRAPHY. I. BASALTIC ROCKS.—A sill of very fresh Markle basalt is well exposed at the northern end of Lunderston Bay, and in a quarry on the hillside. At the latter locality a large dome-shaped" mass of much decomposed, greenish material is seen in the midst of the basalt. This seems to be of the nature of a fumerole, and has induced considerable alteration and decomposition of the basalt in the immediate neighbourhood. The material of the mass must have been highly charged with vapours, the release of which during cooling has caused the alteration of the basalt. The green material is much too decomposed to admit of being sectioned, and it is impossible to determine its exact nature, though the appearance suggests a kind of ash. Similar occurrences are fairly common in the Scottish Car boniferous rocks, several being visible in a small quarry two miles east of Neilston. The basalt consists of large porphyritic labradorite crystals in a medium-grained mesostasis of felspar, granular augite, altered olivine, and magnetite. The phenocrysts show com plicated twinning and well-developed zonal structure. The felspars of the groundmass are mainly plagioclase of the same composition as the phenocrysts—about Ab45 An55—though a little orthoclase is also present. The pyroxene is a greenish non-pleochroic augite, while the olivine is represented by ser- pentinous pseudomorphs. The structure of the groundmass is granular. Two intrusions of basalt occur on the knoll immediately to the north of Gryfe Castle, near Bridge of Weir. The chief evidence of their intrusive nature is the prominent escarpment which surrounds them, and the comparatively coarse-grained groundmass of the rocks. The rock of the north-eastern intrusion, which is near the farm of Yonderton, and which apparently is the one marked in the Geological Survey map, shows-a base of felspar laths, augite and magnetite grains, con taining numerous, somewhat fragmentary, felspar phenocrysts Downloaded from http://trngl.lyellcollection.org/ at Cornell University Library on July 13, 2015 Vol. xvi.] LEITOH—ROOKS OP WEST RENFREWSHIRE—SCOTT. 277 accompanied by smaller serpentinised olivines. The porphy ritic felspars are labradorite of a composition varying from Ao40 An60 to Ab50 An50, while the felspars of the groundmass, which have often an irregular fluxion arrangement, are slightly more acid. Twinning on the pericline law is somewhat common. The olivine is completely serpentinised, and varies in size from the porphyritic crystals down to small grains in the groundmass. The granular augite is of a pale brownish- green colour, and is partly enclosed by the felspar. The rook is allied to the Jedburgh type, but differs from the latter in the presence of the porphyritic felspars. The intrusion which forms the knoll north of Gryfe Castle is composed of a rock which, though resembling the Markle type, differs considerably from the above. The phenocrysts are labradorite, often with numerous inclusions of ferromagnesian material, olivine which is larger and much more abundant than is usual in the Markle basalts of this district, and scarce augite. The olivine is largely replaced by haematite. The mesostasis is fairly coarse-grained, and consists of brownish titaniferous augite and magnetite, sub-ophitically enclosed in felspar laths" which are mainly labradorite and rarely ortho clase. The proportions of ferromagnesian and felspathic minerals are approximately equal. The rock, while allied to the Jedburgh type, has also some affinities: with the Markle basalts, and therefore may be termed a Jedburgh-Markle basalt. A number of isolated dykes are found in various localities throughout the district, and of these the most remarkable is that which occurs at the Lyle Road, between Greenock and Gourock. The strike is approximately north and south, and it can be traced for a distance of about 400 yards. It is exposed immediately to the south of Fort Matilda station, where it is 6 feet in width. Ascending the cliff towards Craig's Top another exposure is found at the side of the Lyle Road. Here the width has diminished to about 6 to 8 inches, and a few feet higher up the last exposure shows a width of 2 inches. Hence we have a case of a regular decrease in thickness as we ascend, and it is probable that the last exposure is within a few feet of the termination upwards of the dyke. Similar phenomena are described by Geikie with regard to the Cleveland Dyke, but in the latter case the thinning out is not to gradual. The Downloaded from http://trngl.lyellcollection.org/ at Cornell University Library on July 13, 2015 278 GEOLOGICAL SOCIETY OP GLASGOW. [Trans. dyke is intrusive into the lavas, and at the Lyle Road exposure the country rock is a fine-grained, olivine-poor basalt with rather acid plagioclase indicating an affinity with the basaltic mugearites. The dyke rock is a Markle basalt of unusual type. It con sists of large phenocrysts of felspar and olivine in a fine-grained groundmass of felspar laths, augite microlites, and magnetite. The felspar phenocrysts are labradorite of intermediate com position and very unusual rhomboid habit. Prism faces are usually absent, and the form development is b(Q10), c(001), and x(101), while the crystals are generally tabular parallel to b(010). The crystals show a paralleled orientation due to fluxion, and the form (010) is generally vertical throughout the mass. Sections perpendicular to b(010) show albite twinning, while complex interpenetration twins are also common. Though the habit is similar, these crystals differ from those of the well- known mineral of the rhomb-porphyry of southern Norway in composition, as the Renfrewshire mineral is labradorite, while the Norwegian crystals are composed of soda potash-lime plagio clase approximating to potash oligoclase. Though the olivine crystals are entirely replaced by serpentine, the reticulate structure of which suggests antigorite, the outlines are per fectly definite, and there is no evidence of any magmatic resorp tion. The groundmass is a closely packed " felt " of pyroxene and felspar microlites with scarcer grains of magnetite. Though the rock is by no means typical of the Markle basalts, it must, on account of the presence of porphyritic felspar and oh vine, be. included in that group. About two miles south-west of Kilmacolm, in the valley of the Blacketty Water, several rocks which may be intrusive are found. There is practically no field evidence regarding their nature, but their freshness, compared with the altered state of the surrounding material and the comparative coarseness of the groundmass, suggests that they are intrusive. One of the occurrences, which is probably a sill of considerable thickness, is found at the southern end of the upper reservoir in this valley. The rock is a basalt with numerous porphyritic felspars, with prominent zoning, set in a base of felspar laths, augite grains and abundant magnetite. The felspar pheno crysts are somewhat more acid than is usual in basalts of the Downloaded from http://trngl.lyellcollection.org/ at Cornell University Library on July 13, 2015 Vol. XVi.] LEITCH—ROCKS OF WEST RENFREWSHIRE—SCOTT. 279 Markle type, while the laths of the groundmass by their uni formly low extinction are probably andesine. Olivine is prac tically absent, and the abundant magnetite is segregated into bands and clots.