342 Scientific Geology. 



which exhibits a double set of these segregated ridges, the smaller 

 ones amounting to fifty-five, and the larger ones not being parallel to 

 one another. I did not notice the direction of the layers in this 

 bowlder, but probably it coincides with that of the most numerous 

 ridges. 



Veins of Segregation in Mica Slate : Chesterfield. 



Mineral Contents. 



More simple minerals occur in this rock than in any other, with 

 the exception perhaps of granite. 



To begin with the earthy salts: it is hardly needful to mention one 

 so common as calcareous spar, which always occurs more or less in 

 connection with limestone. It is in distinct crystals sometimes, also, 

 in the mica slate ; as at Chester, where several of its secondary forms 

 have been noticed. The laminated variety occurs in connection with 

 the micaceous limestone in Whately, Conway, &c. 



The sulphate of alumina and potassa, or native alum, is not unfre- 

 quently found efflorescing upon mica slate ; resulting from the de- 

 composition of the sulphuret of iron and probably also of feldspar, 

 as this is the most probable source from whence it derives the potassa. 

 In Sheffield it is said that " pounds of it can easily be collected in as 

 nearly a pure state as that of commerce."* The localities mentioned 

 in the first part of my Report, in Leominster and Barre, lam satisfied 

 ought to be referred to the gneiss formation; although in hand 

 specimens, the rock in which the alum occurs can hardly be distin- 

 guished from mica slate. 



* Robinson's Catalogue of American Minerals, p. 69. 



