METAMORPHIC ROCKS OP THE OMEO DISTKICT, GIPPSLAND. 221 



some of the present leaders of geological opinion.* In some minor 

 matters, however, I find myself unable to fall in entirely with 

 some explanations of the manner in which the metamorphic 

 changes have been brought about by pressure. The subject is 

 surrounded by enormous difficulties. No direct observation as to 

 the manner in which the schists have been formed are possible, 

 and the sum of experimental knowledge available as a guide to 

 the probable cause and origin of their alteration is but small. 

 The experiments of Daubi^ee, Fouque, and Michel-Seir as to the 

 formation of silicates by the action of heat and pressure upon 

 mineral solutions, or by the fusion of suitable mineral mixtures, 

 seem to me to throw no great light upon the chemical or other 

 processes which mvist have taken place far down within the earth's 

 crust when the crystalline schists assumed the form of mica-schist, 

 or of gneiss. The process which brought about those results 

 must necessai'ily be obscure to us, and our conclusions are there- 

 fore mainly drawn from the results themselves, and from such few 

 experiments as have a direct bearing. 



For instance, the behaviour of silica and of the silicates when 

 under such enormous pressure as that of the earth-stresses 

 referred to, and under raised temperature, can merely, as far as I 

 know, be conjectured. 



The experimental researches of Professor Spring produced only 

 negative results as regards silica when under pi'essui^e not exceeding 

 ID, 000 atmospheres.! But the results of his experiments generally 

 are highly suggestive to the geologist. He found that metals, when 

 under great pressure and in a fairly divided state, formed definite 

 compounds with each other, as for instance copper and zinc, and 

 at the highest pressure employed became viscous, or almost fluid. 

 Alumina under a pressure of 5,000 atmospheres formed a mass 

 resembling halloysite, and at that pressure commenced to become 

 fluid. 



It thus becomes possible to think of the components of rocks 

 under enormous pressure, and heightened temperature reacting 

 upon each other as quasi-solutions, and thus perhaps forming 

 definite mineral combinations. But this conception of the possible 

 results of pressure alone, even if it is that certain substances 

 become solid or fluid, does not shut out the action of solutions in 

 the ordinary sense of the temn. 



It has always seemed to me that in considering what explanation 



*I desire to take this opportunity of pointing' out that my views of the origin of the 

 crystalline schists have underifone considerable change since the time when I first com- 

 menced to study them in the field, and when I described them in 1874 (Quart. Journ. G<:ol. 

 boc, XXXV., p. 1.) My views were then those of Lyell, and I considered that the Omeo 

 schists afforded strong evidence of their truth. Continued investigation has, however, 

 satisfied me that those views must in a great measure be abandoned, and that the opinions 

 now held as lo the origin of the crystalline schists by Lehmann, Geikie, Bonney and others 

 must be adopted, if not in their entirety, yet on the broad lines laid down by them 



t Walter Spring, Recherches sur la propri^te qui poss6dent les corps solides de se souder 

 par Taction de la pression. Bull. Acad. R. t'ciences de Belgiqne, 1880, XLIX.. pp. 

 323-379. 



