42 A TREATISE ON METAMORPHISM. 



error. The importance of the time factor has been recognized by most 

 geologists with respect to erosion and many of the other geological 

 processes, but it is of even greater importance in metamorphism. Most of 

 the metamorphic processes are very slow indeed, but the amount of time 

 available in a single geological period is great, and the metamorphic results 

 are often stupendous. 



In general it may be said that in proportion as rocks are old they are 

 likely to have been greatly altered; in proportion as they are young they 

 are likely to have been little altered. While time is a most important 

 factor in the amount of alteration, time alone, without the other necessary 

 conditions for change, is not sufficient to insure important metamorphic 

 results. Further, when the other conditions are very favorable to change, 

 extensive alteration may take place in" a comparatively short time, consid- 

 ering this factor from a geological point of view. It follows, because of 

 variations in other factors than time, that in some regions very ancient 

 rocks may be little modified and in other regions comparatively young 

 rocks may be greatly modified. 



Environment — J n many cases environment may be important. If the 

 rocks surrounding- a g'iven rock be porous, this condition readily permits the 

 entrance of the agents of alteration — water and gases — and therefore much 

 more profound change may occur than if the rock were surrounded by 

 comparatively impervious material. This is illustrated by the diabase dikes 

 of the Penokee series of Michigan, which where surrounded by the broken 

 rocks of the iron-bearing formation are completely altered, but which 

 where surrounded by the impervious black slates are comparatively 

 unaltered. A further very important factor in environment is the 

 presence of intruded igneous rocks. Igneous rocks, by conduction, may 

 directly heat the adjacent rocks; but of even greater importance is the 

 fact that igneous rocks may furnish solutions to the adjacent rocks or heat 

 the solutions which percolate through them. These illustrations show that 

 the alteration of a rock may be greatly affected by the surrounding rocks. 



Degree of movement — One of the most important of the factors affecting- 

 alterations is movement; indeed, the factor of movement is so important 

 that it has frequently been made a basis for a classification of metamorphism. 

 Changes of rocks take place with comparative slowness under conditions 

 of quiescence and take place with comparative rapidity under conditions of 



