1026 A TREATISE ON METAMOEPHISM. 



Wherever rocks are exposed which once possessed numerous and large 

 openings and which have been in the belt of cementation for a long time, 

 their general cementation gives conclusive evidence of a vigorous past circu- 

 lation. In consequence of the process itself the openings become smaller and 

 smaller; consequently the circulation less and less vigorous until, when the 

 process of cementation is complete or nearly so, the circulation becomes very 

 feeble. Thus in many regions, humid and arid alike, where there has been 

 vigorous underground circulation, and where great quantities of material 

 have been deposited by the aqueous solutions, the circulation now may be 

 relatively unimportant. At many mining districts these conditions now 

 exist. At the time of rapid deposition of the ores there was a pervasive 

 and vigorous circulation which, as cementation continued, gradually became 

 less and less, and finally practically ceased. But after the ground-water 

 circulation becomes feeble or practically stops, in consequence of cemen- 

 tation, earth movements may again fracture the rocks, and thus a new 

 circulation be inaugurated which results in further cementation and perhaps 

 in further concentration of ore. 



In such cases the amount of the present circulation is proportional to 

 the fracturing which has taken place so recently that the openings formed 

 have not had time to be filled again. In contrast with the circulation in 

 formations containing numerous large openings is that in rocks which con- 

 tain few openings larger than subcapillary size — such as shales and the 

 massive igneous rocks. In rocks such as these, unless orogenic movements 

 produce capillary or super-capillary opening's, the circulation may never be 

 vigorous. The existence of impervious formations, either original or pro- 

 duced by the processes of cementation themselves, will be shown later to 

 be very important factors in the general circulation of underground waters 

 and in the localization of ore deposits. 



The aqueous circulation of the belt of cementation for homogeneous 

 mediums is fully discussed on pages 571-589. It is there seen that 

 the movement of the water may be resolved into two components — hori- 

 zontal or lateral movements and vertical movements. In simple cases 

 these components are usually combined in such a way that the lateral com- 

 ponent is continuously in the same general direction, and the vertical com- 

 ponent of the curved path is first downward and later upward. (See figs. 7 

 to 11.) But in areas of marked topographic relief a part of the water may 



