VI.—Devonian Greenstones and Chlorite Schists of South Devon

C. A. Raisin
1889 Geological Magazine  
Miss C. A. Raisin-Greenstones & Schists of S. Devon. 265 prolonged in a south-westerly direction towards Croghan Kinshelagh. A specimen, collected from a spot one mile north of the summit of this mountain, proved, on examination with the microscope, to be a typical epidiorite. The rock forming the summit itself has undergone more alteration. It has a marked foliated character, and consists mainly of chlorite and " felspar-mosaic." Sphene is also present in abundant grains. Specimens of
more » ... e were also collected about a mile N. of Wicklow, on the S.W. side of Croghan Kinshelagh, and from a small patch of greenstone two miles east of Kilcavan. House, to the east of Shillelagh (Sheet 138). (/.)-In only one case was the occurrence of a serpentinous greenstone noted. My attention was drawn to this rock by Mr. Kinahan. A small patch of it occurs about half a mile west of the patch of epidiorite referred to above, two miles east of Kilcavan House, near Shillelagh. Unfortunately I was prevented by lack of time from visiting the locality myself, and the specimen examined was collected by Mr. Clark. The rock is of a variable green colour, and has the characteristically soapy " feel" of a serpentinous rock. A microscopic section discloses the serpentine in colourless layers, associated with grains of opaque iron-ore and a finely granular substance resembling calcite or dolomite. The powdered rock, treated with hydrochloric acid, effervesces only on warming ; it contains, therefore, as we should expect, the magnesian carbonate. The texture of the serpentine is well brought out between crossed nicols. It has rather the " netted " or " bladed" structure of serpentine derived from augite than the " lattice "-structure peculiar to that mineral when produced by the alteration of hornblende. Whatever may be the nature of the alteration, there can be little doubt that we have here the final product of the alteration of a greenstone (dolerite). As we have seen above, the usual course of the metamorphism of the Wicklow dolerites is, first, the formation of a hornblendic rock (epidiorite, hornblende-schist) and, finally, a chlorite-schist; but the case in point indicates that there are exceptions to this rule.
doi:10.1017/s0016756800176290 fatcat:e6z45yooxnfpdb3baqepp4d4mm