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



279 



18. On some cases of the Conversion of Compact ' Greenstones ' into 

 Schists. By T. G. Bonnet, D.Sc, LL.D., F.R.S., F.G.S., 

 Professor of Geology in University College, London, and 

 .Fellow of St. John's College, Cambridge. (Read February 7th, 

 1894.) 



Last year I described some modifications due apparently to pressure 

 in certain basic dykes. 1 I am now able to supplement that account 

 by a few notes on the changes produced by the same agency on a 

 somewhat similar rock which, however, in all probability, was at 

 first in a still more compact condition than any of those ' green- 

 stones.' 



In the year 1880 I noticed, near the path leading from the 

 Bernina Hospice to the Griim Alp (Engadine), a rather small mass 

 of a green schist, so compact as to resemble a slate, associated with 

 a fairly coarse gneiss. Their relations seemed explicable on the 

 hypothesis of either an interstratification of materials or an intrusion 

 of the green rock into the gneiss. Each supposition had its 

 difficulties, and my impression was recorded in the following 

 words : — " I can hardly believe the green rock to be anything but a 

 schist " (meaning, according to my knowledge at that date, a rock 

 originally sedimentary). I took away specimens and examined 

 these afterwards with the microscope. The green rock presented 

 the ordinary structure of a fine-grained schist ; the gneiss gave some 

 indications of a fragmental condition. But as I supposed (as I had 

 been taught) that gneiss also was a metamorphosed sediment, this 

 was not surprising ; still it seemed strange that a rock, so little 

 altered as the schist appeared to be, should be associated with one 

 seemingly so much altered as the gneiss. Accordingly I deemed it 

 best to keep the specimens in my cabinet, and to wait for further 

 light. 



Fig. 1. — Schistose dyke by the path 

 to the Griim Alp. 



2. Schistose dyke. 



and sends small offshoots into the latter rock 



of interstratification has been heightened by the crushing out of 



Gradually suspicions arose 

 in my mind that the green 

 schist must be a crushed dia- 

 base or kindred rock. Hence, 

 on revisiting the Engadine 

 last summer, I took an early 

 opportunity of examining the 

 locality, and was speedily 

 convinced of the correctness 

 of my surmise. A dyke 

 which cuts the gneiss makes 

 a low angle with the horizon, 

 The appearance 



1 Quart. Journ. Geol. Soc. vol. xlix. (1893) p. 94. 



