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PBOCEEDIXGS OF THE GEOLOGICAL SOCIETY. 



perhaps be forgiven for repeating the words I then used. Every 

 rock since its first formation " has undergone and it still is under- 

 going a constant series of internal changes, the result of the action 

 of different causes, as heat, pressure, solution, the play of many 

 chemical affinities, and of crystallographic and other molecular forces, 

 causes insignificant perhaps in themselves, but capable under the 

 factor time of producing the most wonderful transformations. The 

 geologist is called upon to unravel the complicated results, to 

 pronounce what portion of the phenomena presented by a rock is due 

 to the forces by which it was originally formed, and what must be 

 referred to subsequent change ; to discriminate the successive stages 

 of the latter and to detect their various causes ; in short to trace 

 the history of a rock from its deposition to the present moment/'' 



Dr. ^\ T adsworth has well characterized the changes which take 

 place in rock-masses as due to the tendency of unstable mineral 

 combinations to pass into stable ones. It must be remembered, 

 however, that stability is a relative term, and that the arrangement 

 of molecules which is stable under one set of conditions, becomes 

 unstable under another set. As by the internal movements and the 

 external denudation of the earth's crust, the conditions under which 

 rock-masses exist are undergoing slow but continual change, new 

 adjustments of the molecular structure of the mineral elements of 

 such rocks are at the same time necessitated and brought about. 



In attempting to reason as to the original conditions under which 

 a rock-mass must have been formed, it is of great importance to 

 avoid those sources of error which exist in rocks that have under- 

 gone much secondary alteration. Such rocks abound in, though 

 they are not necessarily confined to, the older geological formations ; 

 and it is among the younger and fresher rocks, therefore, that we 

 may most hopefully seek the key to many petrological problems. 



If, for example, we concentrate our attention upon the more recent 

 and less altered igneous rocks, it becomes clear that the degree of 

 crystallization displayed by them has depended on the slowness with 

 which consolidation has taken place, and that this has in turn been 

 determined by the depth from the surface at which they have been 

 formed. In this way, by the study of igneous rock-masses in 

 Scotland and in Hungary, I was able to show that there is a perfect 

 gradation from highly crystalline rocks (granites, diorites, and 

 gabbros) into the ordinary volcanic types (rhyolites, andesites, and 

 basalts, respectively), and from the latter into the various kinds of 

 volcanic glass. These conclusions have been confirmed by subsequent 



