280 PROF. T. G. BONNET ON SOME CASES OP THE [May 1 894, 



the two rocks. The following words written on that occasion 

 describe the macroscopic aspect of the green schist : — " The rock 

 looks just like a slightly altered, poorly cleaved slate, the cleavage- 

 structure making a low angle with the horizon and running nearly 

 parallel with the slight foliation of the gritty-looking gneiss. The 

 schist occasionally (especially in the inner part) seems less fissile 

 and more massive. It has also two sets of sharply-defined joints, 

 making angles of 75° or 80° with the horizon, and is traversed by 

 two or three thin veins of quartz." The thickness of the dyke is 

 rather variable : it hardly ever exceeds a yard, and is generally a 

 foot or so less. 



About a hundred yards farther on is a smaller dyke, about half 

 a yard thick. Here some of the gneiss is much crushed, and the 

 foliation of the two rocks is not always parallel, the structures in 

 one place making an angle of about 20°. llather beyond this comes 

 a third dyke about 8 feet thick, which is less fissile and altogether 

 more like diabase. Others were found in the neighbourhood, but 

 further details seem needless. 



A specimen from the most slaty part of the first dyke (an inch or so 

 from the edge) is seen on microscopic examination to be composed 

 chiefly of three minerals, all minute. (1) A pale green, mica-like 

 mineral in wavy films, slightly dichroic. It is difficult to be sure of 

 the extinction ; in some cases it appears to be parallel with the 

 cleavage, in others slightly oblique. Placed at an angle of 45° with 

 the vibration-planes of crossed nicols the films give fairly brilliant 

 polarization-tints. Some may be a chlorite, but the general aspect 

 of most of the mineral suggests a hydrous ferro-magnesian mica. 

 (2) A rather granular mineral of a pale yellowish colour, which 

 appears to be associated with an earthy dust. Some, at least, of 

 this is probably epidote, but sphene also may be present. (3) A 

 water-clear mineral in rather angular elongated granules. This 

 forms a kind of matrix for the other two, and resembles a secondary 

 felspar rather than quartz. The structure of the specimen is that 

 of a very fine-grained but crystalline schist, and all traces of an 

 igneous origin have completely disappeared. The specific gravity is 

 2-55. 



A specimen, which exhibits a junction with the gneiss, is not 

 quite so distinctly foliated as the former one, especially for about a 

 tenth of an inch near the contact-surface. In this part it is more 

 granular and earthy-looking and is less transparent ; in one place it 

 has a rather streaky structure. The weld between the two rocks is 

 perfect, and the subsequent pressure, as a rule, seems not to have 

 produced separation, except here and there, where a very small 

 crack appears to have formed and to have been filled up afterwards 

 with a greenish mica. 



A slice cut from the middle part of the second dyke mentioned 

 above exhibits a foliated mass of microcrystalline minerals. Among 

 these the micaceous one already described is much the most abundant, 



