RECENT RESEARCHES IN THERMAL META- 



MORPHISM. 



PART I. 



FEW questions, if any, on the physical side of geology 

 have attracted more attention than those connected 

 with the metamorphism of rock-masses ; and, in view of the 

 vast areas occupied in many parts of the world by rocks 

 which either are, or have been considered, metamorphic 

 rocks, it cannot be said that the importance of the subject 

 has been over-rated. Most of our geological text-books 

 divide metamorphism into " local " or " contact," observed 

 in the vicinity of intruded igneous rocks, and " regional " or 

 " normal," more widely distributed and ascribed rather 

 vaguely to various causes. When we come to discuss the 

 probable conditions under which extensive transformations 

 of rock-masses have been effected, this division does not 

 seem to be a philosophical one. The two causes to which 

 geologists have been led in seeking the explanation of the 

 most striking phenomena of metamorphism are high tem- 

 perature and mechanical force, and the true distinction 

 therefore seems to be between thermal and dynamic meta- 

 morphism. As regards structural and, to a less extent, 

 mineralogical changes, the two sets of phenomena are found 

 to be in great part distinct : there must arise, however, 

 cases in which the discrimination presents difficulties, and 

 cases in which the effects of the two causes have been 

 impressed simultaneously or successively upon the rocks of 

 the same area. 



" Contact-metamorphism," due to the heat of an intruded 

 igneous magma, must stand as the typical case of thermal 

 metamorphism, in the sense of offering the simplest and 

 least ambiguous examples for investigation ; but it is 

 not the only case. In the rise of the isogeotherms, as 

 pictured by Babbage, Mellard, Reade and others, extensive 

 tracts of rocks once cool may be directly invaded by the 



