ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 115 



in their appropriate places. Rock-change also acts beneficially 

 in economic geology apart from mineral deposits, as in converting 

 sand into sandstone, limestone into marble, and felspar into 

 kaolin ; but these changes hardly come within the scope of our 

 present subject. 



As these remarks may be read by some who have not as yet 

 entered on the study of Physical Geology, it will be necessary to 

 state a few main facts by way of preparation, and these may be 

 conveniently put in the form of axioms as follows : — 



1. All stratified, and many, if not all, unstratified rocks 

 are largely composed of materials derived from the destruction 

 of pre-existing rocks. 



2. The destruction of these pre-existing rocks has taken 

 place in various ways. Thus, for example, rocks have been (a) 

 worn away and re-deposited, (b) melted up and re-solidified, or 

 (c) dissolved and re-precipitated. Thus rocks such as sandstone 

 and clay, usually termed aqueous, are examples of a ; those termed 

 igneous, such as lava and granite, are examples of b ; while the 

 chemically and organically-formed rocks, such as travertin and 

 coral limestone, are examples of c. Miea-schist, gneiss, and 

 similar rocks have probably been formed by combinations of two 

 or more of these methods of origin. 



3. Rock-masses once formed are subject to continual alter- 

 ation in structure, texture, and composition. Such alterations 

 are effected by mechanical agencies, such as contraction, expan- 

 sion, pressure, and strain ; by chemical agencies, such as solution 

 and chemical affinity; by physical agencies, such as heat and 

 electricity ; and by molecular agencies, such as crystallization and 

 the like. Given these agencies, working together or separately, 

 with sufficient intensity and with sufficient time for their action, 

 and there is no limit to the amount of change that may be effected. 



4. All rocks are more or less permeable to water and other 

 solutions, not only through their joints but also through their 

 substance. 



5. All rocks and mineral substances are more or less 

 soluble in pure water, and still more so in waters containing 

 carbonic acid or other active chemical substances in solution ; 

 such waters, in fact, as are found to circulate in subterranean 

 channels and fissures. 



