Prof. J. P. Cooke on the Vermiculites. 271 



may be the result of a more fundamental and molecular macling, 

 which results when the angle is exactly 120°. 



Observations of Senarmont. — The only previous observations 

 which we have been able to find bearing on the subject of this 

 paper are those of the late eminent mineralogist, H. de Senar- 

 mont, of Paris. In a well-known paper (Ann. de Chim. et de 

 Phys. 3rd ser. vol. xxxiii. p. 391), Senarmont showed that salts 

 which were both geometrically and chemically isomorphous 

 might have very different optical relations — for example, that, 

 while the biaxial crystals of two such salts might have the same 

 bisectrix, the plane of the optical axes in the crystals of one 

 might be perpendicular to the corresponding plane in those of 

 the other. He further proved, by crystallizing together two 

 salts so related, that in the crystals of the isomorphous mixtures 

 thus obtained the optical angle varied with the varying propor- 

 tions of the constituents between the extreme conditions in the 

 crystals of either salt; and by trial he succeeded in forming 

 from two biaxial salts crystals which, in monochromatic light at 

 least, appeared uniaxial. In a later paper (Ann. de Chim. et de 

 Phys. 3rd ser. vol. xxxiv. p. 171) Senarmont applied the principles 

 which he had thus experimentally verified, to explain the varia- 

 tion of the optical angle of the micas. In this paper he seeks 

 to prove, first, that all micas may be referred to a right rhombic 

 prism with angles of approximately 60° and 120°; secondly, 

 that while in some micas the plane of the optical axes is parallel 

 to the shorter diagonal of the rhomb, as in fig. 13, in others it 

 is parallel to the longer diagonal, as in fig. 14. Interpreting 

 these facts by the results of his experiments on isomorphous 

 salts, he draws the inference that there are, crystallographically 

 at least, but two species of mica which are geometrically iso- 

 morphous but optically distinct, that these are represented by 

 the varieties which have the widest optical angle between the 

 axes in either plane, and that all other varieties with optical 

 angles varying from 0° to 70° in either direction are isomor- 

 phous mixtures of the two optically distinct conditions of the 

 mineral. 



The observations described in this paper, although they prove 

 that another cause may also determine the variation of optical 

 angle in micaceous minerals, do not necessarily invalidate this 

 beautiful theory of Senarmont. The variations observed with 

 other minerals, not only on different specimens, but with the 

 same specimen at different temperatures, and which are so beau- 

 tifully seen with the orthoclase from Wehr in the Eifel, and with 

 crystals of selenite, indicate that such variations may be deter- 

 mined by conditions of molecular structure wholly independent 

 of the macling here described. We have shown that the macling 



