

536 



Prof. T. G. Bonney—The Effect of Pressure 



interstratification of more argillaceous and more calcareous layers. 

 It is also evident that these rocks, after they had assumed a crystal- 

 line condition, were modified by a strong pressure, definite in direction. 

 This pressure in many cases appears to have acted at right angles 

 to the planes of the original, or ' stratification-foliation,' and to 

 have superinduced upon it a secondary or ' cleavage-foliation ' in 

 the same direction ; but sometimes, as for instance may be seen 

 at the very summit of the Gorner Grat, the former structure is 

 folded so as to cross the direction of the latter, which then usually 

 becomes inconspicuous. The bedding in these schists dips roughly 

 to the N.W. or a little W. of this, at an average angle of about 40°, 

 but minor disturbances make a very precise determination almost 

 impossible. 



Pyroxenic constituents are generally absent from the serpentine of 

 the Gorner Grat, so far as I have seen, but grains of iron-oxide 

 (magnetite or perhaps chromite) which sometimes attain a fair size 

 are rather common. Thus the rock originally must have been the 



Fig. 2. — Contortions in Slaty Serpentine (natural size). 



variety of peridotite called dunite. Occasionally we find specimens 

 of fairly normal serpentine, but the rock commonly is more or less 

 fissile, looking compressed or even crushed. In some places it is 

 converted into a veritable slate, and the mountain is strewn with 

 slabs, which have smooth level surfaces, and exhibit a structure as 

 compact as that of an argillite, so that their true nature might readily 

 be overlooked. But the more normal and only slightly cleaved 

 serpentine can be traced into this slaty kind, which we then see 

 indicates only localities of greater pressure or of less strength in the 

 rock-mass. In some instances (as near the base of the Riffelhorn on 

 its northern side) the serpentine is so fissile that it can be split into 

 films hardly thicker than an ordinary visiting card. I brought away 

 a specimen, perhaps about half a dozen square inches in area (larger 

 could have been easily obtained), which is nowhere thicker than an 

 eighth of an inch, and in not a few parts is actually translucent. 

 Not far from this place I noticed specimens which seemed to indicate 

 that the rock had been twice affected by pressure, for the thin slaty 

 layers were bent into a series of V-like folds, like a row of gables, 

 perhaps half an inch high, and rather less than an inch wide, these 

 bent layers being separable one from another (Fig. 2). 1 



1 A very fine specimen, on a scale about five times that figured above, was found 

 last summer at tbe top of the Theodule Pass by Prof. W. Kanisay, F.R.S., aud given 

 to our Museum at University College. 



