ORIGIN OF ZEOLITES IN FRESH ROCK. 163 



processes of rock- weathering and rock decomposition, and also the prob- 

 able efficacy of these zeolites as conservators of potash in such form that 

 it becomes available for plant food. It is unnecessary to here go into a 

 discussion of the literature or even to more than briefly refer to the 

 causes which have led to these conclusions. The fact that soda and 

 potash can, in laboratory practice, be made to mutually replace each 

 other in gelatinous precipitates of aluminum or magnesium, as well as 

 in true zeolitic compounds, taken together with the ready decomposa- 

 bility of zeolites by weak acids and the presence in soils of easily de- 

 composable compounds of these bases, are the main grounds for the 

 assumption. So far as the writer is aware, no one has as yet actually 

 proved either the formation of zeolites during the process of true weath- 

 ering or of their existence in the weathered product in other than frag- 

 mental forms as detrital products from the breaking down of zeolite- 

 bearing rocks. 



As yet much is to be learned regarding the formation of zeolites. So 

 far as can be judged from available information, they form most readily, 

 if not invariably, in the presence of continual moisture and where pro- 

 tected from oxidizing influences. Synthetic experiments have shown 

 that zeolites may be formed in sealed tubes during a comparatively brief 

 period at temperatures of not above 150 to 200 degrees centigrade. The 

 inference is fair that, given a longer time, they will form at lower tem- 

 peratures, provided, of course, other conditions are favorable. In looking 

 over the literature of the subject, in connection with my work on rock- 

 weathering, I have, however, become impressed by the fact that a possi- 

 bility has been too generally accepted as a probability, and, further, that 

 the terms weathering, decomposition, and zeolites have been used so loosely 

 that their exact significance is lost, and statements made are therefore 

 frequently either misleading or incomprehensible. 



Use of the Term Weathering. 



First as to the use of the term weathering. It is evident at once that 

 this name should include only those physical and chemical changes in 

 rock masses induced by atmospheric agencies, by temperature changes, 

 and by the chemical action of water and absorbed gases and salts. It is 

 a process which works from above downward, and which manifests itself 

 mainly in a physical disaggregation of the rock and a hydration and oxi- 

 dation, often accompanied by solution of its various constituents. Such 

 action presumably can be carried on to a depth dependent upon the 

 permanent water level, where it practically ceases. Below this level hy- 

 dration and other chemical changes may be going on whereby new min- 

 erals are generated, but the final results are of a quite different nature. In 



