071 the Genesis of Some Scottish Minerals. 203 



form of metamorphism of old sediments. Those who held 

 that view attributed the change from the one type to the 

 other to the action of hydro-metamorphism — that is to say, 

 these geologists recognised water as the most important 

 factor concerned. Even those who no longer adhere to this 

 latter view still regard water as one of the factors concerned 

 in the genesis of eruptive rocks, though they are, perhaps, 

 disposed to regard its presence as incidental rather than as 

 essential. 



Every student of geology has long been aware of the fact 

 that enormous quantities of aqueous vapour are given off 

 during volcanic eruptions. And, again, most chemists 

 recognise that eruptive rocks have been formed, at least, in 

 the presence of either water itself, or else of its constituents, 

 in some form. 



Nevertheless, there are at present very few geologists, and 

 probably still fewer petrographers, who would be prepared to 

 go further in the direction indicated by these facts, and who 

 would be willing to consider whether, after all, eruptive 

 rocks may not have arisen mainly through the action of 

 water. It is quite true that no one is yet in a position to 

 express positively an opinion either for or against any of the 

 currently received views, and it is with some diffidence, 

 therefore, that I venture to put forward an hypothesis relat- 

 ing to this very much disputed subject of the origin of the 

 eruptive rocks, which is based upon the assumption that 

 they are deposits from aqueous solutions. 



The facts which, in addition to those already mentioned, 

 have led me to formulate these conclusions are not now 

 generally called in question by field geologists. Foremost 

 amongst these stand a large number of cases connected with 

 the mode of occurrence of intrusive masses, which demon- 

 strate that these intrusive rocks do not displace the rocks 

 they invade, but actually replace them, as if by some process 

 of pseudomorphism. That is to say, where a given thick- 

 ness of intrusive rock occurs, an equivalent volume of the 

 rocks invaded has in some way disappeared. For example : 

 if we take the case of two coal-seams which are normally 

 separated by 100 feet of other rocks of sedimentary origin, 



