GEOLOGICAL FACTORS AFFECTING ROCK ALTERATION. 41 



Hence porosity is favorable to rapid change; density is favorable to 

 stability. 



water and gaseous content — j n proportion as rocks contain water and gas 

 they are readily altered. In proportion as water and gas are absent they 

 are stable. 



climatic and geographic conditions — The speed of alteration of rocks is affected 

 by their geographical position. The alteration of surface rocks is more 

 rapid in tropical than in arctic regions; it is more rapid in humid than in 

 arid regions; it is more rapid on steep than on gentle slopes; it is more 

 rapid along coasts than in the interior. In short, the nature of the altera- 

 tions of the upper belt of rocks varies with every varying factor of climate 

 and geography. 



Time — Time is a factor of the very highest importance in metamor- 

 phism. Time can not be included among the forces or the agents of meta- 

 morphism, but the amount of metamorphism is a function of the time. 

 Where a given set of forces and agents is at work under a given set of 

 conditions, increase of time increases the metamorphism, but not in a direct 

 ratio, for in proportion as adjustment to environment is approached the 

 alterations decrease in speed. The importance of time in geology can not 

 be too strongly emphasized, for a comparatively weak force or agent 

 working through a great length of time may accomplish an almost incred- 

 ible amount of work. We are accustomed to judge of the efficiency of a 

 force or agent by observations in the chemical or physical laboratory, but 

 the time through which an experiment may be continued in the laboratory 

 is an idhaces^ infinitely small fraction of the time through which the forces 

 and agents have been at work in nature. To illustrate, in the chemical 

 laboratory the amount of crystallized silica which can be dissolved in water 

 and transported to another place within the time during which an ordinary 

 experiment is carried on is so small as to be immeasurable, and yet it is 

 certain that in nature water has dissolved and transported to other places 

 enormous quantities of silica. (See Chapter VII, pp. 622-623.) This illus- 

 tration enforces the fact that the geologist has very much more time at his 

 command than has the chemist or the physicist. If the geologist ignores 

 this fact, and reasons in reference to the potency of forces and agents in 

 metamorphism as a chemist or physicist would in the laboratory in refer- 

 ence to the same forces and agents, he is certain to fall into very serious 



