VII. Regional metamorphism

Abstract

Metamorphic rocks hare heen divided into two classes—1. Those in which local changes have heen caused by contact with 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 “ Metamorphism." 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 worh 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 tha t 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 contactor 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 for the fusion of the volcanic rocks, bu t 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. M allet’s experiments was to ascertain