S^O T. (i. BONNET ON THE SERPENTINE AND 



rock varies in the baud specimen from a black, schistose, but not 

 very fissile rock, which appears to consist mainly of hornblende, to a 

 dark greyish granitoid rock, in which the hornblende is not con- 

 spicuous, and the prevailing minerals are quartz and felspar in 

 va liable quantities, so that the rock sometimes might almost be called 

 a quartzite ; at others it resembles a vein granite. The more schistose 

 hornblendic varieties, on a closer examination, show, generally, fine 

 white specks of felspar. Now and then we find porphyritic varieties, 

 sometimes with felspar crystals about | inch long, sometimes with 

 hornblende about the same size ; occasionally also the rock appears 

 to contain a talcose or chloritic mineral in large quantities, especially 

 when it shows signs of decomposition. I have examined micro- 

 scopically three varieties of this rock — one (1) a typical specimen 

 of the prevalent black hornblende schist*, another (2) a dark grey 

 granitoid variety, and the third (3) a greyish quartzo-felspathic rock, 

 very difficult to distinguish from true vein granite. 



1. About two thirds or more of the field is occupied by crystalline 

 grains of hornblende of rather irregular outline, with very characteristic 

 cleavage, of a green colour, and strongly dichroic. There are many 

 small rounded or vermicular grains of a black mineral, probably 

 magnetite, often included in the hornblende, occasional small 

 rounded or subangular grains of clear quartz, and a few small 

 acicular hexagonal crystals, probably apatite. The rest of the field 

 is occupied by a kaolinized or altered felspathic mineral, which 

 seems, in parts, as if it had never been perfectly crystallized. 



2. Taken from a junction with a gabbro vein, and shows traces of 

 foliation parallel to the common surface. Exhibits an imperfectly 

 crystallized and decomposed groundmass, as in the last, though much 

 more abundant, as well as numerous fairly defined felspar crystals, 

 generally plagioclase ; quartz rare ; magnetite less abundant than in 

 the last, as is, of course, the hornblende ; the crystals of this are also 

 more irregular, with less-distinct cleavage-planes, often somewhat 

 acicular, platy or fibrous. There are microliths — some, probably, 

 apatite ; others, shorter and strongly dichroic, may be tourmaline. 



3. A rarer variety, closely resembling a vein granite, being highly 

 crystalline, and consisting of quartz, felspar, and a little mica. A 

 microscojtic description of one of these will be given further on. 



In general the hornblende schist, except in the darkest and most 

 compact varieties, shows distinct signs of stratification ; sometimes 

 thin felspathic (or quartzose) and hornblendic bands alternate, 

 occasionally exhibiting current-bedding; and sometimes quartzo-fel- 

 spathic strata, from an inch to a few feet thick, alternate with more 

 hornblendic, chloritic, or earthy layers. Epidote is occasionally 

 present in minute quantities ; and veins of quartz, felspar, and 

 ferruginous matter occur. Eoliation seems generally almost, or 

 quite, parallel with the stratification ; there are many extremely 

 beautiful small contortions. The rock is rather sharply jointed, and 

 weathers into bold headlands, dark in the wash of the waves, grey 

 and lichen-covered where exposed to the blasts. 



* See Mr. Hudleston's analysis, p. 928. 



