Prof. J. P. Cooke on the Vermiculites. 261 



The theory which I have formed to account for these facts is 

 as follows. We may conceive that the molecule of mica is an 

 ellipsoid, whose elliptical section, through the longer axis, can 

 be inscribed in the rhomb of 60° and 120°. Assume now that 

 these molecules have polarity, the rhombic prism would be the 

 normal result of their association, as represented in fig. 9. We 

 may, however, conceive that three of these molecules may become 

 associated by alternate single poles to form a nucleus, such as 

 represented in fig. 10. Such a group once formed would be in 

 a condition of great stability, resulting from the concurrent 

 action of the several poles; and if now each of the molecules 

 developed into a crystal, the result would be a made of a form 

 which is very common in the mineral kingdom. If six molecules 

 united in a similar way to form a nucleus, as in fig. 11, we should 

 also have stable equilibrium (although less firm than before), 

 and the result of a symmetrical development would be a made 

 such as has been represented in figs. 7 and 8. Whether this 

 more complex arrangement is necessary in order to explain the 

 phenomena presented by the vermiculites and micas I do not 

 feel confident. It is seldom that more than three individuals 

 can be distinguished on a given plate; and the very unequal 

 development of the several individuals and the indefiniteness of 

 the lines of demarcation, resulting from the phenomena which 

 have been described, render what would seem to be a character- 

 istic feature of the more complex group not necessarily a certain 

 indication of the structure. I refer to the fact very constantly 

 noticed, that the plane of the optical axes is parallel to the 

 nearest hexagonal edge, as shown in figs. 7 and 8. In figs. 10 

 and 11 this same plane is parallel to the shorter axis of the 

 ellipse ; and it can easily be seen that, if either of the individuals 

 of fig. 10 were developed over any large portion of the space of 

 its neighbours, the optical plane might appear parallel to the 

 adjacent edge. 



Having made the two suppositions as above to explain the 

 phenomena of twinning, which have been long familiar and ex- 

 ternally visible, it will not, we trust, appear unreasonable if we 

 make a third supposition to explain the phenomena first described 

 in this paper. We may conceive that, the ellipsoidal molecules, 

 instead of grouping together on the same plane, become asso- 

 ciated by their alternate poles, one over the other, as represented 

 in fig. 12. Molecules so associated, developing laterally, would 



the relative position of the laminae injures the result to a far less degree 

 than the least inequality in their thickness. It still remains to make care- 

 ful quantitative measurements of the effects produced under determinate 

 conditions; and, as can easily be seen, the subject opens a wide field for 

 mathematical analysis as well as physical investigation. 



