ADAPTATION TO ENVIRONMENT SLOW. 35 



Material may be intruded, or may be brought in by water solutions, or 

 may be abstracted by water solutions. By any one of these processes or 

 by any combination of them a considerable change in the chemical 

 composition of a rock may take place. 



While it may be safely asserted that all rocks, under all conditions, 

 at all times, are being adapted to their environment, the change in a rock 

 goes on so slowly that its lag behind the change in the environment may 

 be measured by millions of years. Often the lag is so great that the con- 

 ditions again change before the process of adjustment has made much 

 advancement, and, therefore, before one set of changes is near completion 

 another set is begun. Indeed, a later change in conditions may be a 

 reversal of an earlier change, and, therefore, in the process of adaptation, 

 work done in an early stage may be reversed at a later stage. But even in 

 such a case it is clear that the principle of adaptation applies, just as in the 

 case of many plants and animals, although there may be, in fact, little more 

 than a tendency toward adaptation to existent conditions. 



Because the adjustment of rock to environment is so slow, in order that 

 it may be approximately complete it is necessary that a rock remain under 

 substantially the same conditions for a very long* time. This has happened 

 in some regions in which important mechanical movements have not 

 occurred for a period or an era and the rocks of which have remained 

 buried to a moderate depth for most of the time. Such were the conditions 

 of the ancient volcanics of certain parts of the Lake Superior region. These 

 have escaped important mechanical movement since the beginning of Pale- 

 ozoic time. They were buried under Paleozoic sediments to a moderate 

 depth. Denudation since the beginning of Cretaceous time brought them 

 to the surface. Finally glacial erosion removed a skin of weathered 

 material and exposed the volcanic rocks, approximately adapted to their 

 past environment, that of the belt of cementation under cjuiescent condi- 

 tions. (See Chapter VII, pp. 594 et seq.) So far as they have reached the 

 surface they are subject to a new set of conditions; and a new cycle of 

 change, begun at the end of the Glacial epoch, but not far advanced, is in 

 progress. 



In considering metamorphism, the fundamental hypothesis of geology 

 will be applied as in other branches of the subject. That is to say, 

 the Huttonian principle, that the present is the key to the past, is 



