86 PROCEEDINGS OF THE AMERICAN ACADEMY 



The results of analysis in column 1, and the portions of the descrip« 

 tion of the mineral in quotation-marks, above, have been taken from 

 " Dana's System of JMineralogy," fifth edition, page 493 ; and the atomic 

 ratio which is there deduced is, 



IV VI II II 



Si : B : B : U = U : A : 7 : 5. 



In this analysis, however, Crossley could not have determined the state 

 of the iron, which, in the specimen I have examined, is almost wholly 

 in the ferric condition. If now we assume that the whole of the iron 

 belongs with the sesquioxide radicals, the analysis would appear as in 

 column 2, and the atomic ratio is then seen to be 2:1:1:1, which is 

 undoubtedly the correct result. 



In the year 1851, Mr. W. W. JefFeris, of West Chester, Pa., discovered 

 at the ripidolite locality near that town a peculiar micaceous mineral 

 which exfoliates like the Millbury vermiculite, but wliich, instead of 

 occurring in small foliar, 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 (Am. Jour. Sci., 11. xxxi. 3G9, 

 1861), was subsequently described as a new species (Am. Jour. 

 Sci., II. xli. 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 micaceous 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 connection 

 with other minerals from his corundum mine on the Culsagee River, in 

 Macon County, N. C, a specimen of still another micaceous mineral 

 having the same remarkable pyrognostic properties. It proved to be 

 the best defined of any of this class of minerals which I had examined, 

 and I shall designate it as Culsageeite. 



Besides the above, there have been found several other micaceous 

 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 mineral under consideration undergo when heated 

 are analogous to the well-known phenomena presented in the dehydra- 

 tion 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 



