Page:The Journal of geology (IA journalofgeology11893univers).pdf/148

The researches of numerous geologists during the last two decades have placed at our disposal a large amount of information respecting the metamorphism of rocks, and from the facts thus collected we are now in a position to draw conclusions which we may expect to have a wide application. The important changes that affect the character of rock-masses divide roughly into two classes.

First, there are those dependent on meteoric agencies. These changes, though not necessarily superficial in the ordinary sense, are due in the first place to the action of circulating waters in communication with the atmosphere, and as a rule they involve the addition or subtraction of various ingredients or the transference of material from one place to another. The ordinary "weathering" effects illustrate the removal of alkalies and silica, the addition of water, oxygen, carbonic acid, etc. We must also include the processes which have given rise to many crystalline limestones and quartzites, serpentine-rocks, dolomites, iron-stones, and jaspers, and even (as appears from Van Hise's researches in the Penokee region) some mica-schists and fine-grained gneisses. The characteristic of almost all these transformations is that they are metasomatic as well as metamorphic.

Secondly, we have those transformations more usually understood by the term metamorphism: viz., dynamic metamorphism, due to high pressure operating upon rock-masses, and thermal metamorphism, due to high temperature, whether produced by an intrusion or by the mechanical generation of heat. In these various cases of metamorphism proper, metasomatism is rather the exception than the rule. I shall deal here with thermal metamorphism only, and shall draw my data chiefly from the rocks surrounding the large igneous intrusions of the English Lake District, investigated by Mr. Marr and myself, but the conclusions are confirmed in other areas.