Prof. J. P. Cooke on the Vermiculites. 267 



over the basal edge of about 122°, resembling the crystals of 

 Biotite from Greenwood Furnace. Mr. Hall informs me that 

 these more perfect crystals have only been found in one pocket 

 of the serpentine. 



The distinction, however, between the phlogopites and the 

 Biotites is not fundamental, either chemically or physically. 

 Chemically, both species are orthosilicates ; that is, the atomic 

 ratio between the silicon and the sum of the basic radicals is 1 : 1. 

 The species differ in composition only in the relative proportion 

 of the sesquioxide and protoxide radicals. In the phlogopite 



II VI 



the ratio of R to R is probably normally 2:1; but of the pub- 

 lished analyses the value varies between that ratio and the ratio 

 3:2. In the Biotites the same ratio is probably normally 1:1; 

 but here, again, the different analyses which have been made give 

 values varying between 5 : 3 and 1:2. In like manner the op- 

 tical distinction between the phlogopites and Biotites, of which 

 so much has been made, is equally indefinite. Between a so- 

 called phlogopite like that from Jefferson County, N. Y., with 

 an angle of about 15°, and the apparently uniaxial plates of 

 Biotite from Vesuvius, there is every possible gradation — some- 

 times, as I have shown, on one and the same mica plate ; and I 

 have endeavoured in this paper to explain the cause of this va- 

 riation. With the Vesuvian Biotites themselves (if the specimens 

 in the mineralogical cabinet of Harvard College are fair repre- 

 sentatives of the mineral from that locality) it is only occasion- 

 ally that we find a perfectly uniaxial plate. More commonly 

 there are distinct evidences of twinning ; and on the borders of 

 the hexagonal plate may be discovered a biaxial structure, of 

 which the optical plane is parallel to different edges of the 

 hexagon on different parts of the plate. 



It must, however, be remembered that as, by the process of 

 twinning we have described, the structure of the magnesian micas 

 approaches that of uniaxial crystals, rhombohedral and other 

 planes 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. 

 In other words, the process of twinning we have illustrated in this 

 paper produces hexagonal crystals in external form as well as in 

 optical characters ; and the question naturally arises, May not 

 the hexagonal crystals of other minerals be formed in a similar 

 way ? that is, may they not be developed from twinned mole- 

 cules, which, though in their aggregate producing an hexagonal 

 structure, singly would develop into biaxial crystals ? Bearing 

 on this point we have discovered some very remarkable evidence. 



