36 A TREATISE ON METAMORPHISM. 



assumed. Where certain phenomena are now produced by certain com- 

 binations of forces and agents, and by these only, and similar phenom- 

 ena are found in the rocks long- since formed, it is assumed that the like 

 phenomena, present and past, are due to essentially the same combina- 

 tions of forces and agents. For instance, if alterations of a certain 

 kind are now being produced by a complex set of geological factors, and 

 by these only, where similar alterations are found in ancient rocks it is 

 assumed that they are due to practically the same combination of the 

 forces and agents of alteration. 



But the above statement does not imply that the changes are now 

 taking place with the same speed as that with which they occurred in the 

 past, as might have been held by Lyell; nor is it assumed that the various 

 forces and agents have the same relative values. Indeed, it is believed to 

 be highly probable that there have been changes in the rate of alteration 

 of rocks and in the nature and effectiveness of the factors producing the 

 alterations. 



While the Huttonian principle is of service in the study of metamor- 

 phism, the alterations of rocks take place so slowly that it does not have 

 nearly the value that it has in the study of the work of the epigene agents, 

 such as air and water and ice; nor the value it has in the study of such 

 hvpogene agents as volcanoes and earthquakes. 



Many of the rock alterations are now taking place under conditions 

 which can not be directly observed, but must be inferred from the records 

 of the change. This statement applies to all changes below a mile in 

 depth, and it is very largely applicable to all but the mere outer film of the 

 rocks, for most excavations and cuttings are not deeper than a few score or 

 a few hundred feet and the deepest shafts are but little over a mile. For- 

 tunately it frequently happens that in a rock formation now at the surface 

 the results of various stages of change under deep-seated conditions are 

 preserved, so that the character of the alterations and the nature of the 

 forces and agents which have produced them may be inferred from a close 

 study of the different stages of alteration. 



In such cases, instead of observing the forces and agents accomplishing 

 certain results, and of reasoning that similar results produced in the past 

 are due to these forces and agents, we observe the results at various stages 

 of development and infer from them the nature of the forces and agents 

 producing them; we then infer that similar forces and agents are at work 



