364 PROF. T. G. BONNET ON THE GAENET-ACTINOLITE [Aug. 1 898, 



III. Results of Miceoscopic Examination. 



As these rocks are well known to petrologists, and their minerals 

 are seldom, if ever, exceptional, I will nob lengthen this paper by 

 giving minute descriptions of individual specimens, but will briefly 

 notice the chief constituents, and then discuss certain structural 

 peculiarities which appear to throw light on the past history of the 

 group. 



The specimens, when examined by the microscope, exhibit, as 

 we might expect from macroscopic inspection, certain features in 

 common, together with considerable variety in constituents and 

 structure. All show a cleavage-foliation, but some less distinctly 

 than others ; all contain nearly the same minerals, but differ in the 

 amount of garnet, of mica (both biotite and paragonite), and of horn- 

 blende, as to both size and arrangement. In all a kind of ground- 

 mass is present, consisting of a water-clear mineral, with more or 

 less of a silver- white micaceous constituent. The latter is generally 

 subordinate to the former, though once or twice it predominates. 

 In this groundmass are scattered iron-oxide, garnet, staurolite, horn- 

 blende (rarely absent), and biotite (in larger flakes). The water- 

 clear mineral in the groundmass forms a kind of mosaic, the grains 

 varying from subrotund to angular, and seldom exceeding -01 inch 

 in diameter. They are well cemented together, showing no sign of 

 recent fracture or crushing, but sometimes contain minute enclosures 

 irregularly disposed, like a little dust. They generally resemble 

 quartz and give, with crossed nicols, similar colours. Some may 

 be this mineral, but the majority are undoubtedly felspar, a few 

 exhibiting twinning on the albite-type. Two or three, which 

 occur in small veins, or are in contact with calcite (an occasional 

 constituent), are partially idiomorphic. The micaceous constituent 

 has been identified as paragonite by previous observers. It occurs 

 in small colourless flakes, often with rectilinear outlines, generally 

 from about '003 to '007 inch in length. In one or two specimens, 

 as we should expect from their macroscopic aspect, it is so abundant 

 that in places the water-clear mineral can be distinguished only in 

 minute interstitial granules. This mica generally exhibits more or 

 less of a foliated structure, and sometimes, where abundant, appears 

 to be crumpled. Though iron-oxide and biotite might be occasionally 

 claimed as constituents of the groundmass, I think that they are 

 more properly reckoned with the minerals which have a porphyritic 

 habit. These are as follows : — 



(1) Iron-oxide, which is not very abundant, occurs in granules 

 or grains, and in fairly well-formed flattish crystals, the mode of 

 association of the first suggesting that they are the detritus of a 

 larger grain. The mineral here and there is faintly translucent, 

 having a deep reddish-brown to brownish-red colour. Some of it 

 may be magnetite, but most is haematite, perhaps also ilmenite. 



(2) Garnet (not in every slice) of a pale pinkish-red tint, vtdth 

 enclosures of iron-oxide and other small minerals, and with cavities, 



