or Saussurite-smaragdite Gabbro of the Saasthal. 245 



We may now pass on to inquire to what cause the slight 

 foliation, commonly perceptible in one variety of this rock 

 (No. 4), should be attributed. 



The smaragdite-euphotide, it may be remarked, is a rock of 

 exceptional hardness and toughness, breaking, apparently, 

 with equal difficulty in all directions ; but the variety of 

 which we speak is not quite so intractable, and is occasionally, 

 though not always, more fissile in the direction of its streak- 

 ing. But, so far as I have observed, we do not find any 

 surfaces of markedly imperfect cohesion, such, for instance, as 

 are common in the " augen-gneisses."" A careful, and I hope 

 unprejudiced, examination of the smaragdite-euphotide blocks 

 convinced me that they fail to reveal any conspicuous marks of 

 ct dynamo-metamorphism.' > '' In many, notwithstanding the 

 changes each constituent has undergone, an ophitic structure 

 on a large scale is still quite clear. The more granular 

 varieties, whether coarser or finer, resemble the more granular 

 types of ordinary gabbro. 



Here it may be well to describe in some detail two or three 

 examples, which appeared to me to give important evidence as 

 to the history of the rock. First of these is one, which, at 

 the first glance, might be supposed favourable to the idea of 

 dynamo-metamorphism. It is a large block (about 6 feet by 3| 

 and at least a yard thick), consisting of saussurite and a dull 

 green, almost dark-slate-coloured hornblende, which is slightly 

 more porphyritic in aspect than is usual, crystalline grains of 

 " saussurite" about 1 inch or 1*25 inch long occurring in a mass 

 where they ranged downwards from about "5 inch. These larger 

 crystals have a rather irregular outline. They are not, how- 

 ever, oval as in " augen-gneiss/" but the ends are a little 

 ragged or " teased " out, the smaller being more streak-like 

 (fig. 2) . Thus the rock exhibited a slight 

 foliation, but was not in the least fissile -p^„ 2. 



parallel with it ; the structure in short to 

 my eye suggested fluxional movement c£T^7 <! ^J 

 prior to solidification rather than a crush- c^? 



ing of a mass already solid. Here and £T^3 

 there smaragdite comes in, grains of it <-^~^^ 



being associated with and apparently 

 replacing the dark-bluish (hornblendic) mineral. These grains 

 occur in cloudlike streaks or veins, which have a slight tendency 

 to run parallel with the structure of the rock. 



Another large block, the face of which was about 5 feet wide 

 by 3 feet high, which lies some distance farther up the valley, 

 also affords important evidence. At the top is a band, about 

 6 inches high, of a coarse gabbro, containing both smaragdite 



