DOLEKITE INTO HORNBLEXDE-SCHIST. 141 



the intervening material lost : but the two portions are represented 

 in their natural position, and the zone of transition is fortunately 

 preserved in the upper one. A large section prepared through the 

 zone of transition shows in a very perfect manner its gradual 

 character. 



In what way was the foliation produced ? The only answer that 

 seems possible is, that it was caused by movement when the mass 

 was in a plastic state ; but plasticity may arise in two ways. 

 Under moderate pressures it can only occur when the material is in 

 the fluid condition, that is, when the cohesion is slight ; but under 

 very high pressures it may be produced in bodies which are solid 

 under ordinary circumstances *. This notion of the plasticity of 

 solids is, of course, one of immense significance in relation to rock- 

 deformation, and is doubtless destined to play a most important 

 part in all questions relating to the origin of the crystalline schists. 

 The phenomena of the Scourie dykes appear to me to show that the 

 plasticity which has led to the development of foliation, is that due 

 to high pressure at ordinary temperatures, rather than to high tem- 

 peratures at ordinary pressures. 



The optical anomalies due to strain, the bending of the twin 

 lamellae, and the actual fracture of the felspar crystals of the 

 dolerite, are best explained on the assumption that the rock has 

 been affected by pressure after consolidation, and the same assump- 

 tion explains the curved and confused arrangement of the joint- 

 planes which is occasionally observed. Then the fact that the 

 planes of foliation frequently cut across the dyke and run parallel 

 with the prevalent strike of the gneissic banding, is difficult to 

 account for on the view that the foliation was produced by 

 movement during the final stages of consolidation, though easy to 

 explain on the alternative hypothesis. The curious interfelting of 

 gneiss and dyke at the junctions in certain places also appears to 

 indicate movement posterior to consolidationf. 



Mechanical deformation accompanied by molecular rearrangements 

 has profoundly affected the gneiss itself; and I have little doubt 

 that some of this has taken place since the intrusion of the dyke. 



mechanical action alone, others that have been formed by mechanical action 

 supplemented by a certain amount of moleculai' rearrangement, and others, again, 

 like the hornblende-schist of the Scourie dyke, in which a complete, or nearly 

 complete, molecular rearrangement has taken place. The data for the solution 

 of the above problems are therefore in all probabilitv to be found in this region. 



* Tresca, " Flow of Solids," Proc. Inst. Mech. Eng. 1878, p. 301. 



i" It is interesting to note that if this process of interfelting were carried on 

 to a much greater extent, it would lead to the disappearance of the dyke as such. 

 The only evidence that would then remain to attest the former existence of the 

 dyke would be the presence of bands of hornbleiide-schist in the gneiss. We 

 know nothing as to the age of the dyke except that it is later than the so-called 

 Archgean gneiss. It is somewhat interesting to contemplate the possibility of a 

 later djke of unknovni age becoming part and parcel of Archaean gneiss ; and 

 the reflection may serve to suggest that our gneisses and schists may be of very 

 complex origin, and may contain, as Prof Lapworth holds, elements derived from 

 formations of very different geological age. 



Q.J.G.S. ^0.162. M 



