366 PEOP. T. Gr. BONNET ON THE GAENET-ACTINOLITE [Aug. 1 898, 



dark variety, and is generally rather irregular in outline, as if a 

 crystal or grain had been broken up and re-cemented.^ 



(4) Biotite is always present, though the quantity is very 

 variable, and three types may be distinguished : (a) one, perhaps 

 the least frequent, in small flakes, somewhat irregular in outline, 

 in rather streaky groups ; (b) small, fairly regular flakes, associated, 

 as described below, with large crystals of actinolitic hornblende ; 

 (c) large flakes, with well-defined but not crystalline outlines, clear 

 and fresh-looking. The first type has probably been formed by 

 the breaking up of a larger fiake under pressure ; the second I 

 pass over for the moment, as it can be most conveniently discussed 

 in connexion with the hornblende. The third type occurs in 

 comparatively large flakes, say up to ^ inch broad, is strongly 

 pleochroic, and contains granules of iron-oxide and the water- 

 clear mineral of the groundmass, sometimes with small flakes of a 

 rather pale green mica. The last pierces the biotite at various angles 

 with its basal plane, sometimes also is included in it, and occasionally 

 is quite separate (see fig. 5, p. 365). This mineral usually is free 

 from inclusions, and rather feebly pleochroic, changing from a pale 

 greenish straw-colour to a light dullish green. I have no doubt that 

 it is a hydrous biotite, for flakes of that mineral sometimes are in this 

 condition at their edges. But, from the way in which it occurs as 

 an inclusion in the larger anhydrous mineral, it must either diff'er 

 in chemical composition (so as to be more readily afi'ected by water) 

 or (as 1 think more probable) it must be an older mineral, which 

 was hydrous at the time of inclusion. The flakes of ordinary biotite 

 lie with their basal cleavages in various directions, perhaps more 

 often than not at a high angle with those of cleavage-foliation, and 

 the facts mentioned above lead me to infer that they were formed 

 at a comparatively late date in the history of the rock. 



(5) Hornblende. A smaL grain of rather fragmental aspect may 

 be occasionally seen in the groundmass, but this mineral generally 

 occurs in fairly definite crystals. jS'ow and then (especially in one 

 specimen) these are almost idiomorphic ; they are always more or 

 less lancet-shaped and sometimes acicular,^ the outlines in the 

 latter case being always rather irregular. They are in colour a 

 rather rich green, and strongly pleochroic. They include grains of 

 iron-oxide, of the water-clear groundmass, and well-defined small 

 flakes of biotite. In some cases we find a number of parallel prisms 

 of hornblende, like an imperfect grating or gridiron, separated by 

 larger intervals of a water-clear felspar or even by a mosaic of the 

 same, as if an effort had been made at crystal-building, which had 

 only produced a skeleton. The more elongated forms often exhibit 

 a fringing growth of small flakes of biotite, or rather pass into this 

 externally, so that the flakes conform very nearly to the outline 



1 The occurrence of staurolite in a rock which probably has been in a 

 molten condition is unusual, so far as "we know, but a parallel may be found in 

 andalusite. 



^ The 'needle,' it must be remembered, is a stout one, often quite a thick 

 bodkin. 



