222 JPogue, Jr. — Geology and Structure of Volcanic Rocks. 



from 1 : 1 to 6 : 1. This excess of soda in a sedimentary rock is 

 very unusual. For comparison, an average was made of the per- 

 centages of alkalies found in the 33 slates analyzed in the 

 chemical laboratory of the United States Geological Survey 

 from 1880 to 1903,* with this result: 



Average JS"a 2 O=:0-89 per cent. 

 " ' K 2 0=3-68 



The ratio of soda to potash is 1:4-1. Only two of the 33 

 slates fail to have at least twice as much potash as soda ; and 

 in no instance is the soda in excess. These figures show that 

 the material of a normal slate, during an ordinary cycle of land 

 erosion, loses soda much more readily than potash ; so that the 

 final result is a preponderance of potash over soda, irrespective 

 of the original proportions. When the reverse is found to be 

 the case, special conditions must be sought to explain this 

 unnatural relation. 



In the slates of the Carolina slate belt, the soda is equal to 

 or in greater amounts than the potash. According to the 

 quantitative nomenclature, the Carolina slate is sodipotassic to 

 dosoclic ; whereas, a normal slate is dopotassic. This feature 

 indicates that the rock has not undergone a normal cycle of 

 erosion ; for such would have brought it in line with the 

 average slate. On the contrary, it suggests that the original 

 material of the rock was transported only a short distance, 

 and, further, that the material was presented to the transport- 

 ing agent in a condition of mechanical disintegration. A long 

 transport of finely comminuted material would have resulted 

 in the deposition of sediments low in soda. A long period of 

 chemical weathering, previous to transportation, would have 

 had the same effect. The conclusion, which is strengthened 

 by the geologic occurrence, microscopic make-up, and grada- 

 tion into tuff dej3osits, is that the slates were derived chiefly 

 from great masses of volcanic ejectamenta, and deposited by 

 water, with varying amounts of land waste, at no great dis- 

 tance from the source of the material. 



The acid series of volcanic rocks. Acid fine tuff. — The 

 acid fine tuff occurs interbedded with the slate and the acid 

 coarse tuft, and is transitional into each. It has no wide-spread 

 areal extent, but is abundantly distributed in very narrow 

 lenses, often represented by single outcrops. These are often 

 intimately associated with outcrops of the acid coarse tuff, and 

 the two form tuff bands parallel to the belts of slate. At 

 times there are frequent alternations in the course of a few 

 yards between fine tuff, coarse tuff, and slate, bespeaking a 

 rapid change of conditions during deposition. 



* Clarke, F. W., Analyses of Kocks. U. S. Geol. Survey, Bull. 228, 

 pp. 337-346, 1904. 



