26 METAMORPIIISM. 



might have occurred, either by the different degrees of fusibility in the materials, their different compos! I ion, 

 or the greater or less amount of heat introduced into them. 



The above facts and reasonings authorize a more sweeping conclusion, viz. that almost 

 every rock is capable, by metamorphism, of being converted into almost any other. It is 

 usual to suppose that we are to find in the metamorphic rock only the ingredients that 

 exist in that from which it was derived. But if the latter be made plastic by aqueo-igneous 

 agency, why may not the water present contain other ingredients not in the original rock ? 

 And who can set limits to the varieties of rocks that might then be produced ? 



In view of such facts, also, we can readily assent to Bischof s conclusion, when he says, 

 " The mineral kingdom, therefore, contains nothing that is unchangeable, unless perhaps 

 it be the noble metals gold and platinum." 



A third important agency in metamorphism is the atmosphere. Its four constituents, 

 nitrogen, oxygen, carbonic acid, and aqueous vapor, all act upon the rocks, not merely at 

 the surface, but by means of water they are carried deep into the earth, and furnish prob- 

 ably a large part of the chemical agents that are active in metamorphism. Thus nitrogen 

 and oxygen uniting, form nitric acid, and nitrogen combining with the hydrogen resulting 

 from organic changes, forms ammonia ; and both of these agents, nitric acid and ammonia, 

 carried by water into the crust of the earth, form very energetic agents of change, we 

 know not how deep. Carbonic acid, also, is soluble in water, and is thus introduced 

 among the rocks, which it dissolves by direct action and by uniting with other ingredients 

 to form other re-agents. There is enough in the atmosphere to contain 2,800 billion 

 pounds of carbon, and this carbon acts as a carrier of the atmospheric oxygen, first intro- 

 ducing it among the plants and rocks as carbonic acid, and leaving it by other combina- 

 tions to escape again. These atmospheric agents operate quietly, but the amount of 

 disintegration exhibited almost everywhere by the rocks shows that the work is a mighty 

 one. The atmosphere, which, as we breathe it, seems so bland and inefficient, is in fact silently 

 crumbling down the solid rocks we know not hoAv deep with a power compared with 

 which the efforts of the quarry man and the miner are mere infinitesimal blows. 



A fourth metamorphic agency at work in the earth is galvanism. All chemical changes 

 do, indeed, imply the presence of this force : but we know of no other agency which in 

 rocks but partially plastic could transfer ingredients from one part of the mass to 

 another, as seems to have been done, and to be now doing. Thus, a vein of copper ore 

 has been divided by a transverse crack so that the two ends were separated some inches. 

 But the fissure was subsequently filled with sand ; and after some years it was found that 

 the vein was continued across the opening by the introduction of copper ore. Again, 

 how, but by galvanism, can we explain the production of cleavage, foliation, and joints ? 

 These have required a polarizing force, and galvanism^is such a force. 



FOKMEB PLASTIC CONDITION OF THE ROCKS. 



Such are the chief- agents of metamorphism. Let us now proceed to consider the most 

 important effects they have produced in the rocks. 



In the first place, they have brought most of the stratified rocks into a plastic or semi-plastic 

 condition, subsequent to their original consolidation, and continued them in that state for a great 

 length of time. 



