1885.] 



Regional Metamorphism. 



425 



VII. ''Regional Metamorphism." By Joseph Prestwtch, M.A., 

 F.R.S., Corr. Acad. Sci. Paris, Professor of Geology in the 

 University of Oxford. Received June 11, 1885. 



Metamorphic rocks have been divided into two classes — 1. Those in 

 which local changes have been caused by contact w T ith heated eruptive 

 rocks; 2. Those extending over wider areas, in which the rocks are 

 in no apparent relation to eruptive or igneous rocks. The first has 

 been termed Contact Metamorphism, and the second Normal or Regional 

 Metamorphism, the latter two terms having been used to express the 

 same phenomena and treated as synonymous. 



The object of this paper is briefly to show that there may be another 

 cause for metamorphic action, for which, not to introduce a new term, 

 I would propose to transfer and restrict the term of " Regional Meta- 

 morphism" Normal metamorphism I would confine to signify, as 

 hitherto, the changes caused by the heat due to depth, on the supposi- 

 tion of the existence of a heated central nucleus of the earth, while 1 

 would use the term regional metamorphism to denote changes effected 

 by the agency of the physical causes to which Mr. Mallet referred the 

 fusion of the volcanic rocks, namely, the heat produced locally within 

 the crust of the earth by transformation into heat of the mechanical work 

 of compression, or of crushing of portions of that crust * 



I was led to consider the importance of this action by the abnormal 

 result presented in the distribution of the underground isotherms in 

 the St. Gothard Tunnel, and which on looking into the question can 

 only, as it seems to me, be attributed to the residual heat arising from 

 the crushing of the rocks during the upheaval of that portion of the 

 Alpine range, which is of very late geological date ; and also by some 

 cases in which the alteration in the rocks hardly seemed explicable upon 

 the hypothesis either of ordinary contact- or normal-metamorphism. 



This other source of heat had not been altogether overlooked by 

 geologists, though only occasionally referred to as a secondary cause ; 

 but its actual importance had hardly been realised until Mallet inves- 

 tigated the subject experimentally and mathematically. He failed to 

 show sufficient cause lor the fusion of the volcanic rocks, but he drew 

 attention to the enormous heat-producing power of certain earth 

 movements. This power, inadequate though it may be to explain the 

 phenomena of vulcanicity, is singularly applicable in explanation of 

 some of the metamorphic phenomena exhibited in mountain ranges. 

 The object of his experiments, however, having been to establish the 

 maximum results to be attained by the force of compression, only 

 bears indirectly on the collateral problem we are here considering. 



The primary object of Mr. Mallet's experiments was to ascertain 



* " Phil. Trans. Roy. Soc." for 1873, p. 147. 



