Geological Society, 



155 



the indices of refraction of red light, the brightest part of the spec- 

 trum, and of violet light were about 1*890, 1*973, and 2*143 re- 

 spectively. 



Carbonate of Thallium. 



The faces which have been observed are all in one zone, and 

 exhibit a symmetry which is compatible with either the prismatic 

 or the oblique system. The crystals probably belong to the pris- 

 matic system. They are aggregated in such a manner as to render 

 it very difficult to isolate a single crystal, or to determine the faces 

 which belong to the different individuals of a group of crystals. 



Observed forms :— 1 0, 1 I 0, 2 1 0, 1 2 0. Fig. 4. 



1 0, 1 1 

 1 0, 2 1 

 10 0, 12 

 1 1 0, I 1 



51 28 

 32 7 

 68 57 



77 4 



Twins. Twin face 110. One individual is generally united to 

 each of two others, in this respect resembling the twins of cerus- 

 site, aragonite, glaserite, and chrysoberyl. 



A cleavage has been observed probably parallel to the faces of the 

 form 110; it may, however, be parallel to the faces of the form 1 0, 

 the complexity of the twin crystals being such that it could not 

 be ascertained whether the cleavages observed belonged to one crys- 

 tal or to two different crystals. 



I am indebted to Mr. Crookes, the discoverer of thallium, for the 

 crystals of nitrate, sulphocyanide, and carbonate of thallium, above 

 described. 



GEOLOGICAL SOCIETY. 



[Continued from vol. xxx. p. 453.] 



November 22, 1865. — W. J. Hamilton, Esq., President, in the Chair. 



The following communications were read : — 



1. " On the impressions of Selenite in the Woolwich Beds and 

 London Clay." By P. Martin Duncan, M.B., Sec. G.S. 



Spaces formerly occupied by crystals of Selenite having been 

 described by the author as occurring in Woolwich Beds near 

 Mottingham, Kent, and in the unfossiliferous London Clay of Ten- 

 dring Hundred, he endeavoured to account for the phenomena to 

 which he had drawn attention. The various facts bearing on the 

 question, including the conditions under which the beds were depo- 



