﻿Vol. 64.] EOCK8 OF GLENDALOUGH AND GEEYSTONES. 491 



lY. The Contact-Phenomena of the Amphibolite. 



The Ordovician sediments into which the peridotite was intruded 

 can be studied for a few feet only on the north-eastern side of the 

 boss and for a few yards on the eastern side. Beyond these limits 

 the ground is lower and covered with turf and fern, and it seems 

 probable that the rocks are softer. When the ground can be 

 studied again at a distance, it consists of silvery phyllites and mica- 

 schists, with occasional lenticles of quartz-schist, the metamorphism 

 of which is ascribed by Prof. SoUas to the contact-action of the 

 Leinster Granite. 



The contact-rocks along the north-eastern margin of the boss 

 present a peculiar gnarled appearance. jSTo lines of bedding can be 

 traced for more than a few inches without interruption. There 

 are in general three kinds of bands, which probably correspond 

 to differences in deposition. These are bands of granular quartz, 

 dense brown bands which prove to consist of minute garnets, and 

 layers which are characterized by the predominance of stout plates 

 of biotite set at right angles to the bedding. The interruptions 

 consist in part of anastomosing veins of quartz, garnet, or biotite ; 

 and in part of a sudden loss of the banded character, and its re- 

 placement by a confused aggregate of coarse and fine patches of the 

 same minerals. The junction of these rocks with the actinolite-rock 

 is sharp. 



In sections, biotite, muscovite, magnetite, chlorite, garnets, quartz, 

 and occasionally felspar, may be recognized. 



The biotite forms the largest crystals. Longitudinal and basal 

 sections possess highly irregular outlines (fig. 9, p. 492). It en- 

 closes garnet, muscovite, magnetite, and occasionally quartz, and is 

 moulded ophitically on these minerals. Pleochroic halos are 

 abundant, many of them round zircon. 



The biotite is partly altered into a pale chlorite, which contains, 

 besides the above-mentioned inclusions of the biotite, large blotches 

 of magnetite. The alteration proceeds along the cleavage in an 

 exceedingly-regular manner, so that the biotite seems to be inter- 

 grown with chlorite. The halos are preserved in the chlorite, 

 retaining their greater pleochroism and a higher birefringence than 

 the surrounding mineral. 



The muscovite is in much smaller individuals than the biotite, 

 generally in rectangular sections, frequently twinned, and grouped 

 in varying ways. In some layers the muscovite forms a felted mass, 

 almost to the exclusion of quartz and garnet. 



The crystals of garnet are often so small, that they may lie 

 wholly within the surfaces of a thin section ; and their perfect idio- 

 morphism may be seen by focussing. They seem to be free from all 

 inclusions but iron-ores. 



The quartz is rarely so fine-grained that the crystal-boundaries 

 cannot be recognized. They are generally polygonal or circular, 

 and never show the interlocking of a dynamically-formed schist. 



Q. J. G. S. No. 256. 2 l 



