ARRANGEMENT OP MICA IN THE BEDS. 



429 



beds as in the dikes, the mineral fragments lie with their long diameters 

 parallel ; furthermore, they are in the bedding plane. The scales of mica 

 are all parallel, and when viewed edgewise, in general arrangement they 

 look like those in the sandstone dikes ; yet there is an important difference . 

 Their parallel arrangement in the two cases is the result of very unlike con- 

 ditions. The particles of sand, subsiding under the influence of gravity 

 alone, lie upon their flat sides, and in this manner all become parallel in the 

 sandstone bed and lie in the plane of stratification. A large particle of 

 mica at first straight lies horizontal, stretching perhaps over several grains of 

 other mineral. Other grains in turn fall upon it, and being pressed down 

 by accumulating sand above, indent or bend the mica and make it conform 

 to the irregular outlines of the adjacent grains as in figure 7. In sediment- 

 ary rocks, where the scales of mica are crushed in the process of deposition in 

 water, the crushing takes place perpendicular to the plane of foliation in the 

 mica, and does not ordinarily produce openings between the folia ; but in 

 the sandstone dikes the mica was crushed parallel to the plane of foliation 

 as represented in figures 5 and 6. 



Chemical Composition of the Sandstone Dikes and Beds. 



Chemical analyses have been made of five specimens of sandstone from 

 dikes which are widely separated and equally distributed throughout the 

 field. The results are as follows : 



* The material of Nos. 3, 4, 5, and 8 was dried at 101° C. 



