I04 PROCEEDIJSGS OF THE GEOLOGICAL SOCIETY. 



granites. The same may be observed sometimes in certain schists ; 

 but the latter commonly appear to have crystallized under pressure 

 more definite in direction than is the case with these contact-meta- 

 morphic rocks. The structure in the schists may often be due to 

 subsequent compression followed by recrystallization, but I doubt 

 whether it is so in every case. In one other respect there is a 

 marked difference between the contact rocks and the older meta- 

 morphic. In the*former, felspar (of endogenous origin) is very rare, 

 if not wholly absent. I have never seen an instance of it that 

 was beyond question (except in the immediate vicinity of the con- 

 tact-surface) ; generally it is wanting. Though often an analysis 

 shows that there is enough of an alkaline constituent left in the mud 

 to make a fair amount of felspar, still this is taken up by mica, and 

 the alumina silicates (or subsilicates) are non-alkaline. 



In this rough sketch I have omitted, for simplicity, several of the 

 mineral changes, such as the formation of garnet, idocrase, mag- 

 netite or haematite, pyrite, graphite, because, mutatis mutandis, the 

 same method of explanation would apply ; or such as the formation of 

 epidote, which, indeed, may also result, like the introduction of 

 carbonates, as a yet further change ; or of tourmaline, so abundant 

 in many of the Cornish specimens, because I regard the last as 

 exceptional and the result of a process of addition possibly subse- 

 quent to the intrusion, since it affects both the rocks. It is intended 

 to show simply how far Nature in that part of her laboratory-work 

 which we are permitted to contemplate, though at a distance, throws 

 light upon that which is only a matter of inference. We see then 

 that there are analogies between the contact-metamorphism and 

 the regional metamorphism ; but the latter has some very marked 

 differences from the former, chiefly in the presence of felspars and, 

 in most cases, in the greater size and definiteness of the mineral con- 

 stituents*. Moreover contact-metamorphism, as we might expect, 

 in its more marked effects, is a very limited phenomenon. Between 

 the so-called schists and gneisses produced by it which I have seen, 

 or between the silky slates, phyllites (or so-called schists) of 

 Palaeozoic or later age, in much-folded districts, and the true schists 

 and gneisses, of either known or presumed Archaean age which I have 

 been able to examine, there is a very wide gulf, which, notwith- 

 standing all assertions to the contrary, I do not believe has yet 

 been filled up by well-established transitional forms. 



Progressive Metamorphism in a downward Direction. 

 It may, perhaps, be urged that, as the evidence of stratification 

 decreases and the amount of mineral change increases as we proceed 



* As these differences are so marked, it would be well to abstain from calling 

 any of the rocks thus produced " mica-schists," &c. Dr. A. Geikie (Text Book, 

 bk. ii. pt. ii. § 7) suggests the reviyal of the old name Cornubianite. This might 

 well be done, provided it be understood that the alumina-silicate was, as a rule, 

 not felspar, but some subsilicate, such as andalusite ; or, having regard to the 

 etymology of the term, and to the fact that tourmaline is very commonly the 

 aluminous constituent of the Cornish " contact" rocks, we might define Cornu- 

 bianiie as essentially consisting of quartz, mica, and tourmaline, and revive the 

 term Proteolite for rocks consisting essentially of quartz, mica, and andalusite. 



