Vol. 50.] CONVERSION OF ' GREENSTONES ' INTO SCHISTS. 283 



lithic. Whether the dominant secondary mineral will be mica or 

 hornblende is probably dependent upon the chemical composition of 

 the rock. If it be a normal basalt, we may expect the latter ; if 

 near the andesites, the former. It is interesting, however, to 

 notice that the ultimate structure of the rock apparently depends 

 to some extent upon the earlier one ; for it is coarser in the thicker 

 than in the thinner dykes, and in the latter becomes distinctly 

 more compact near the margin. The peculiar structure of the 

 junction-specimen : also suggests that it retains a trace of the 

 actual selvage of the dyke. This indicates, by the way, that the 

 structure has been acquired by direct crushing rather than by 

 shearing (in the strict sense of this word), for the peculiar linear 

 banding must have been obliterated, or at least rendered very in- 

 distinct, by any lateral displacements of importance. 



It is also remarkable that this zone — the most glassy or slaggy 

 of the original rock — apparently has yielded less to pressure than the 

 part beyond, for it has a less distinct foliated structure and a 

 stronger resemblance to a compact, somewhat decomposed sedi- 

 mentary rock. 



Kather compact green schists, which not seldom exhibit more or 

 less of a slabby structure, are common in various parts of the Alps. 

 They resemble sometimes the rocks described above, sometimes 



now, after fourteen years of study, after having examined perhaps fifty slices 

 which might be helpful, for every one which I had then seen, I cannot find a 

 better comparison than that the structure of the supposed slate ' resembles 

 that of the groundmass of some of the chiastolite-slates,' though I admit 

 that it is not a good one. We have, in short, been describing specimens which 

 bear little resemblance one to another. 



The most singular feature in my slice is the comparatively slight indication 

 of the effect of crushing. The quartz-felsite exhibits (unless I err) a faint 

 fluxion-structure ; the porphyritic crystals of quartz and felspar are but little 

 broken. In the intrusive rock also the microfohation is not conspicuous till 

 the slice is placed with the structure about halfway between the vibration- 

 planes of crossed nicols, and the gradual change from the selvage to the 

 coarser parts (in a thickness of about one-third of an inch) is quite distinct. 

 Yet the fissile character of the rock, especially near the outside, is most 

 remarkable, and even its texture on a broken edge is not unlike that of a dai-k 

 slate. I have to thank Miss C. Eaisin, B.Sc, for a slide cut from the same dyke 

 (about 4 inches from the margin). In this the microporphyritic character is 

 distinct, but the groundmass more nearly resembles that of my own slide. 

 I am also indebted to her for slices of other dykes in the neighbourhood. A 

 selvage remains in a slaty dyke which occurs on the more western side of the 

 cutting at the northern end of the tunnel, but here no doubt could arise as to 

 the nature of the rock. A conspicuously porphyritic dyke, on the opposite 

 side of the same cutting, also exhibits a selvage. The base of this is of a 

 dark green colour, and it remains dark between crossed nicols : the larger 

 crystals in it are but little affected. There must be something exceptional in the 

 effect of pressure on a compact and rather basic igneous rock. It looks as if 

 the microiuineralogical structure was developed without much fracture of the 

 glassy material. Perhaps this is due to the intimate mixture of the chemical 

 constituents, so that the process is more like that of devitrification. 



1 This remark applies also to the Welsh specimen mentioned in the last note. 



