434 Scientific Intelligence. 
Chemically, both species are orthosilicates; that is, the atomic 
ratio between the silicon and the sum of the basic radicals is 1: 1, 
e species differ in composition only in the relative proportion 
of the sesquioxide and protoxide radicals. In the phlogopite the 
vi 
ratio of & to Ris probably normally 2: 1; but of the published 
Vesuvius, there is every possible gradation—sometimes, as I ha 
shown, on mi 1 d I have endeavored 
in this paper to explain the cause of this variation. he 
mineral from this : 
perfectly uniaxial plate. More commonly there are distinct ev! 
dences of twinning, and on the borders of the hexagonal plate 
It must, however, be remembered that, as by the process of 
twinning we have described the structure of the magnesian micas 
lanes characteristic of the hexagonal system begin to appear on 
the crystal. This is illustrated not only by the crystals of Biotite 
from Vesuvius and from Greenwood Furnace, N. Y., but also by 
the more perfect crystals of Hallite from Chester County, Pa. 4 
as — 
of the magnesian type; and this 
which the lamination is parallel 
