24 J. G. Goodchild — Augen- structure and Enqytive Rochs, etc. 



This is a necessary consequence of crystallization following a 

 gradual increase of temperature which falls short of the degree 

 requisite for the fusion of the most refractory minerals in a rock 

 compound. It may come into play as much in a rock heated by 

 contact metamorphism as in one whose temperature is raised by 

 dynamic causes. I have in mind some cases I have lately seen in 

 which thick piles of pyroxene-labradorite rocks, which were 

 originally lavas, dykes, and sills, have been invaded by extensive 

 protrusions of rocks of acid composition at some period subsequent to 

 the outpouring of the lavas. The result, it appears to me, is that 

 the parts of the lavas which were the last to consolidate from the 

 original molten condition, that is to say, the glassy magma and the 

 pyroxenic constituents, were afterwards fused again in the reverse 

 order of their original cooling, as the temperature of the invading 

 mass began to affect that of the older volcanic rocks around it. But 

 as the melting point of both the felspars and the remaining rock 

 constituents was never reached during the invasion, these minerals 

 remained as they were. Eventually the temperature declined, the 

 pyroxene and the minerals composing the original glassy magma 

 crystallised out, and, around the contact zones, the lavas, dykes, and 

 sills reconsolidated as holocrystalline rocks with ophitic structure, 

 and with a plutonic instead of volcanic facies. 



Analogous changes to these must, I conceive, frequently have 

 taken place in connection with rocks undergoing plutonic meta- 

 morphism ; only in these cases the partial rearrangement of the 

 constituents was due to the heat generated directly by dynamic 

 causes, and as a consequence of the gradual and partial relief of 

 pressure while the rocks in question were in a potentially molten 

 condition. 



The nature of the minerals resulting from any given change of 

 this kind is absolutely dependent upon the nature of the constituents 

 present within segregating distance of the zones affected. If, for 

 example, alkalies are deficient during any given stage of meta- 

 morphism, neither the felspars, nor any other naineral into whose 

 composition the alkalies enter, can result. This has alwaj^s been 

 the great stumbling-block in the way of those who have attempted 

 to account for the origin of gneiss, for example, from metamorphosed 

 sediments. It is certain, from the very nature of the case, that the 

 alkalies must have been removed from the constituents of the 

 sedimentary rocks long prior to the date of their rearrangement in 

 a stratified form. It has very properly been objected to the extreme 

 views held by metamorphists that no kind of fusion or rearrange- 

 ment of any kind is competent to metamorphose an ordinary grey- 

 wacke into a granite, for the simple reason that the most important 

 constituent of one of the essential minerals of the granite, to wit, 

 the potash of the felspars, did not originally occur in the material 

 acted upon. Yet the study of highly metamorphosed rocks whose 

 sedimentary origin is beyond a doubt, shows that, by some means or 

 other, secondary felspar has been formed within such rocks, and in 

 those in which the effects of dynamo-metamorphism has been carried 



