PRINCIPAL AGENTS CONCERNED IN. 75 



Summing up our knowledge of this metamorphism or 

 transformation of mineral matter by which chalk, for 

 instance, can be changed into crystalline marble, and clay 

 into glistening roofing-slate it may be safely affirmed that 

 the following are the principal agents concerned in its 

 production, even though we may not be always able to 

 determine their precise modes of operation : 1. Heat by 

 contact, as when an igneous mass, like lava, indurates, 

 crystallises, or otherwise changes the strata over or through 

 which it passes. 2. Heat by transmission, conduction, or 

 absorption, which may also produce metamorphism, accord- 

 ing to the temperature of the heated mass, the continu- 

 ance of the heat, and the conducting power of the strata 

 affected. 3. Heat by permeation of liot water, steam, and 

 other vapours, all of which, at great depths, may produce 

 vast changes among the strata, when it is recollected that 

 steam under sufficient pressure may acquire the temperature 

 of molten lava. 4. Electric and galvanic currents in the 

 earth's crust, which may, as the experiments of Mr "W. Fox 

 and Mr R. Hunt suggest (passing galvanic currents through 

 masses of moistened pottery clay), produce cleavage and 

 semi -crystalline arrangement of particles. 5. Chemical 

 actions and reactions, which, both in the dry and moist 

 way, are incessantly producing atomic change, and all the 

 more readily when aided by an increasing temperature 

 among the deeper-seated strata. 6. Mechanical -pressure, 

 produced by the mere weight of superincumbent strata, and 

 which is obviously concerned in the solidifying, compact- 

 ing, and hardening of all rock-matter, whether belonging 

 to the superficial or to the deeper-seated formations. 7. 

 New molecular arrangement by pressure and motion a 

 silent but efficient agent of change, as yet little under- 

 stood, but capable of producing curious alterations in inter- 

 nal structure, especially when accompanied by heat, as we 



