32 JOURNAL OF THE 



rite zones. On the other hand, we find many outcrops 

 that present none of these evidences of shearing-, but 

 are perfectly massive and structureless; and we must 

 conclude that these represent the least altered condi- 

 tion of the rock. The massive character of the perid- 

 otites, with often scarcely a trace of shearing*, is whol- 

 ly incompatible with the theory that they are of con- 

 temporaneous orig-in with the g-neiss and have passed 

 throug-h the same cycles of disturbance. 



The line of separation between the peridotites and 

 the g-neiss is always sharp; there are no transitional 

 forms, either in chemical or mineralog^ical composition. 

 The peridotites are extremely basic mag^nesian rocks 

 (40-45 per cent, silica); the constituents of the gfneiss 

 are all aluminous minerals, and the rock is hig-hly acid 

 (60-70 per cent silica). That such dissimilar rocks 

 would have been deposited as contemporaneous sedi- 

 ments or precipitates over wide areas without some- 

 where producing* an intermediate type is, at least, 

 hig-hly improbable. 



Even a mere casual examination of these rocks with 

 the microscope reveals a thoroug*hly crystalline g*ranu- 

 lar structure, g*enerally a coarse texture, and in all re- 

 spects the characteristics of a deep-seated, ig-neous 

 rock. In fresh specimens the olivine g*rains always pre- 

 sent ang-ular outlines, sometimes having* crystal form, 

 and always fitting* perfectly tog*ether, without intersti- 

 tial spaces or cementing* material of any kind. (PI. V., 

 fig*. 1). In the first stag*e of alteration to serpentine 

 the grains are separated by thin films of this mineral; 

 but the irreg*ularities of adjacent g*raius still remain 

 perfect counterparts. (PI. V, fig*. 2.). Only in the 

 more advanced stag*es do the corners become rounded 

 and the intervening* serpentine assume the appearance 



