Vol. 54.] SCHISTS OF THE ST. GOTHARD PASS. 369 



and, in some instances, to occur whe re the included felspar-granul 

 are more abundant than is usual. Hence I infer that we may 

 account for them in the following way : — Whenever hornblende and 

 felspar, especially if intimately mingled, were in contact and in a 

 rather unstable condition,^ the ferromagnesian constituents of the 

 former, and the alkaline-aluminous of the latter, might be com- 

 bined to produce biotite, for the last-named is often the offspring 

 of the other two minerals.^ The larger biotites, however, may 

 result from the reunion of the minerals of an original but crushed- 

 up crystal. The frequent occurrence of flakes, the basal planes of 

 which make high angles with the cleavage-foliation, suggests that 

 often the}' did not begin to form till the pressure had been relaxed. 

 The more or less actinolitic hornblende may be thus explained. 

 This shape, as I have already shown, is commonly assumed when 

 the mineral crystallizes under a pressure definite in direction.^ 

 But in this case something more has to be explained. Acicular 

 or lancet-shaped crystals, often 2 inches long, occasionally more, 

 form bunches, which sometimes diverge from a centre in opposite 

 directions (see figs. 2-4, pp. 362-363). The finest examples of 

 this structure lie in the planes of cleavage-foliation, though 

 some occur, especially if the crystals are more lancet-shaped, 

 with different orientations, probably when that foliation is not 

 very distinct. A single instance may suffice to present the 

 problem to be solved. A flattish crystal of hornblende, about 

 ^ inch wide, splits up into a group like the sticks of a partly- 

 opened fan : the ends of the outermost, at the distance of an 

 inch or a little more from the beginning, being a good | inch 

 apart (and sometimes even more), while a considerable space on 

 either side is quite free from hornblende. Was the powder of that 

 mineral spread rather abundantly over an area shaped like an 

 acute-angled triangle, or, if it were more uniformly disseminated 

 through the mass, could it be forced to concentrate along certain 

 lines ? Which mode of distribution is the more probable? Suppose 

 an ordinary, moderately coarse diorite to be simply crushed ; then 

 a number of patches of powdered hornblende and felspar will be 

 formed, rudely ellipsoidal in shape and elongated considerably in 

 the direction of least resistance.* If shearing also occurs, the result 

 will be somewhat similar, but the patches will be much longer. 

 They will be drawn out in streaks, perhaps with more mixing 

 of constituents. This would explain the coincidence of long 

 actinolites with the cleavage-foliation, but not their occurrence 

 in fan-shaped groups or the rather frequent presence of fair-sized 

 crystals at high angles with that structure. Moreover, so far as 



^ This might depend on differences of temperature in the mass or in the 

 amount of water present. 



^ That this does occur has been already shown (though in that case the 

 circumstances were different) : Hill & Bonney, Quart. Journ. Geol. Soc. vol. xlviii 

 (1892) pp. 127-132. 



3 Quart. Journ. Geol. Soc. vol. xlix (1893) p. 94 & vol. 1 (1894) p. 279. 



* As in the case of ordinary cleavage. 



2d2 



