jPogue, Jr. — Geology and Structure of Volcanic llocks. 221 



In general, each formation lias a massive and a mashed or 

 schistose phase, with every gradation between the two. The 

 passage of heated solutions affected all formations, as evidenced 

 by the mineralized zones, the abundance of quartz veins, and 

 the high degree of silicification in many belts of rock, and 

 the universal occurrence of infiltrated iron ores. Finally, 

 erosion has planed off all the upper portion of the folded series ; 

 but weathering has proceeded in excess of erosion to such an 

 extent that the region is now deeply decayed, so that only 

 here and there do the rocks project through a thick mantle 

 of decomposed rock or soil. 



This threefold set of changes undergone, each cumulative in 

 its effect, has given rise to an almost infinite number of rock 

 variations; but a consideration of the rocks which have suffered 

 the least dynamic and chemical change renders it possible to 

 establish a definite number of distinct, though related, rock 

 types, to which the more altered derivatives maybe referred. 



Slate. — The rock here designated as "slate" is not a normal 

 product of land erosion, but bears evidence of a peculiar origin 

 by a liberal admixture of fine-grained volcanic ejectamenta. It 

 occurs in broad bands, with a northeast trend, separated from 

 each other by belts of volcanic rocks. It shows upon the sur- 

 face as low, elongated, parallel reefs or ledges. These are 

 never very abundant nor continuous, because the rock easily 

 weathers to a soil. Associated with the slate are rather 

 frequent outcrops of the acid and basic tuffs ; the slate in 

 many places grades imperceptibly into the acid fine tuff. 



When fresh the slate is a dark green, dark to light blue, or 

 grayish black to black rock. With increasing proportions of 

 ash, these colors grade into lighter shades, and finally into 

 light grays and whites. Upon weathering, the colors brighten 

 and become quite diverse and sometimes even brilliant. 

 Shades of purple, blue, green, red, yellow, and gray, in endless 

 variations, may appear. In texture, the rock is so dense that 

 little can be discerned with the unaided eye. In many occur- 

 rences bedding planes are visible, bespeaking the sedimentary 

 origin of the rock. Much of the slate is massive, but in many 

 portions of the district it has been mashed to a greater or less 

 extent, so that it splits easily along certain directions. Some 

 of the mashed phases also show bedding planes ; these only in 

 part agree with the schistosity. The rather anomalous term 

 "schistose slate" seems appropriate for some phases of the 

 rock. 



The most interesting and significant feature in regard to the 

 slate is the relation between its soda and potash content. 

 Several analyses show that the ratio of Na 2 to K 2 varies 



