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



lite, but which, instead of occurring in small laminae, is found in 

 large hexagonal plates. This mineral was analyzed by Professor 

 Brush, and, although at first referred by him, " with a query," 

 to vermiculite (Amer. Journ. Sci. (II.) vol. xxxi. p. 369, 1861), 

 was subsequently described as a new species (Amer. Journ. Sci. 

 (II.) vol. xli. p. 248, 1866), and named Jefferisite. 



Several years later, Mr. John Hall, now of Philadelphia, sent 

 to me for examination some rough six-sided prisms of a mica- 

 ceous mineral which he had discovered at East Nottingham, in 

 Chester County, Pa. This mineral also exfoliates when heated. 

 It is a new species ; and I have named it, after the discoverer, 

 Hallite. 



A year since I received from Colonel C. W. Jenks, in con- 

 nexion with other minerals from his corundum mine on the Cul- 

 sagee river, in Macon County, N.C., a specimen of still another 

 micaceous mineral having the same remarkable pyrognostic pro- 

 perties. It proved to be the best defined of any of this class of 

 minerals which I had examined ; and I shall designate it as Cul- 

 sageeite. 



Besides the above, there have been found several other mica- 

 ceous minerals whose pyrognostic and crystallographic characters 

 indicate that they belong to the same family, but which have 

 not yet been investigated. 



The remarkable exfoliation and great apparent increase of 

 volume which the class of minerals under consideration undergo 

 when heated are analogous to the well-known phenomena pre- 

 sented in the dehydration of alum, borax, and other crystalline 

 salts when heated in a similar way; and it will be one object of 

 this paper to show that the effect is due to the same cause — 

 namely, to the escaping of what we call water of crystallization. 

 I also expect to show that the several minerals referred to above 

 are members of a family of hydrous silicates closely allied and 

 parallel to the well-known family of anhydrous silicates called 

 the micas, and that their molecules differ from those of the mag- 

 nesian micas chiefly in containing a definite number of molecules 

 of water — that is, water of crystallization. I shall call this family of 

 minerals " the vermiculites," using the original name, as "mica" 

 is now employed, to designate a class ; and I shall call the three 

 species (or varieties ?) of this family Jefferisite, Culsageeite, and 

 Hallite, which correspond, as I expect to show, to the two varie- 

 ties of Biotite and to phlogopite respectively. It will appear 

 that the original vermiculite has the same composition as the 

 variety from the Culsagee mine. Finally, attention will also be 

 asked to some unexpected discoveries to which the optical ex- 

 amination of these minerals has led. 



Water of Crystallization and Water of Constitution. — In the 



