288 



EEPORT OF NATIONAL MUSEUM, 1899. 



up, and interrupted here and there iDy belts of later formation. Mica veins are found 



iTere, in fact may be said to char- 

 acterize this horizon everywhere, 

 from its eastern outcrop, near the 

 seaboard, to and quite under the 

 flanks of the Smoky Mountains. It 

 is, however, in the great plateau of 

 the west, between the Blue Ridge 

 and the Smoky, that the mica veins 

 reach their greatest development, 

 and have given rise to a very new 

 and profitable industry, — new and 

 at the same time very old. 



It may be stated as a very gen- 

 eral, almost universal, fact, that the 

 mica vein is a bedded vein. Its 

 position (as to strike and dip) is 

 dependent on and controlled by, 

 and quite nearly conformable to, 

 that of the rocks in which it occurs, 

 and hence, as well as on account of 

 their great size, some observers, 

 accustomed to the study of veins 

 and dikes and the characters of 

 intrusive rocks in other regions, 

 have been disposed to question the 

 vein character of these masses at 

 first. But a good exposure of a sin- 

 gle one of them is generally suffi- 

 cient to remove all doubt on this 

 score. The mica vein is simply and 

 always a dike of very coarse granite. 

 It is of any size and shape, from a 

 few inches — generally a few feet — 

 to many rods (in some cases several 

 hundred feet) in thickness, and in 

 length from a few rods to many 

 hundred yards, extending in some 

 cases to half a mile or more. The 

 strike, like that of the inclosing 

 rocks, is generally northeast, and 

 the dip southeast, at a pretty high 

 angle; but they are subject, in these 

 respects, to many and great local 

 variations, all the conditions being 

 occasionally changed, or even re- 

 versed. An idea may be formed of 

 the coarseness of these veins from 

 this statement, that the masses of 

 cleavable feldspar and of quartz 

 (limpid, pale yellow, brown, or, 

 more generally, slightly smoky), 

 and of mica, are often found to measure several yards in two or three of their dimen- 

 sions, and weighing several tons. I have a feldspar crystal from one of these mines 



