16 THE ORIGIN AND ALTERATION OF ROCKS. 



elements of the entire rock mass, and all elements brought into it ])y (he 

 percolating waters ; the chemical reactions taking place between any or iill 

 of these elements according to the special conditions, and not being confined 

 to simple interchanges between the constituents of two minerals, as i:)seudo- 

 morphs in mineral veins are usually explained. The failure to appreciate 

 the above distinctions is believed to have led to the statement of much that 

 is improbable in the works of many writers on pseudomorphic and metauior- 

 pliic changes in rocks. 



When we consider the petrographical structure of modern volcanic 

 districts and the alterations their rocks have undergone, we ouij-ht not to be 

 surprised at the magnitude of the changes which we find to have taken place 

 in rocks which have been subjected to similar conditions during countless 

 ages. But these changes are metamorphic, and the rocks thus altered are 

 metamorphic rocks. Metamorphism, however, does not appear to be lim- 

 ited to rocks of one kind, but affects all classes. The amount of metamor- 

 phism any rock undergoes under the same conditions seems to be inversely 

 proportional to the amount of contained silica ; and this change apparently 

 began as soon as any of the earth's solid material was exposed to the com- 

 bined action of air and water, and has continued up to the present day. 

 Volcanic or eruptive action, including a subsequent prolonged exposure to 

 hot water, accompanying the dying eruptive force, appears to have been nn 

 efficient agent in metamorphism. 



According to the above view, the metamorphic rocks produced would be 

 dependent upon their chemical composition and the agency bj which the 

 changes were effected, but would not be at all dependent upon the geolog- 

 ical age. Hence lithological characters would be valueless as a criterion 

 for determining the age of such rocks. 



The writer finds that the constituents of the eruptive rocks and their 

 derivatives pass in their alteration from the unstable towards more stable 

 compounds in the conditions to which they are subjected, — that is, they 

 pass into forms that never can in the ordinary course of nature return to 

 their original condition. In this there exists a potent factor for the dissipa- 

 tion of energy. The potential energy of the original chemical combination 

 IS in a greater or less degree lost, and cannot be restored except by some 

 foreign power, — or, in other words, the original structure and composition 

 cannot be normally regained. The advocates of the sedimentary origin 

 of igneous rocks, however, require the restoration of that lost energy, and 



