44 PROCEEDINGS OF THE AMERICAN ACADEMY 



optical angle, observed by Descloizeaux on the minute scales of the 

 Pontivy mica, as sufficient ground for separating the new mineral from 

 the old species. We should include under this species all hydrous 

 micas which are rendered by the basic hydrogen orthosilicates, and to 

 it the name Damourite belongs by priority. Sterlingite is simply a 

 variety of Damourite, having the ratio 1:3:4, with more marked 

 qualities and a wider optical angle than the Pontivy mineral ; and, 

 provisionally, the name I have given will be useful in desiguating it. 



The hydrous micas, of which Sterlingite is a variety, have a special 

 interest in connection with the subject of this paper, because they 

 illustrate the characteristics of basic water, which will be contrasted, 

 with those of water of crystallization, in our description of the fol- 

 lowing species. The evidences that the water in these micas is basic, 

 — that is, forms a part of the basic radical, — may be summed up as 

 follows : — 



1. The amount of water in the different varieties is very variable, 

 and bears no constant ratio to that of the other basic radicals.* 



2. The hydrogen of the water supplements the other basic radicals 

 and fills out the amount required for a unisilicate, the type to which 

 most of the micas conform. 



3. The water is expelled only at a very high temperature. 



4. The loss of water is not attended with any marked change in the 

 appearance of the mineral.f 



Jefferisite, of West Chester. — This well-known mineral, found 

 in the serpentine at West Chester, Pa., was, as I have said, care- 

 fully analyzed by Professor Brush, of New Haven, who named it after 

 W. W. Jefferis, Esq., of West Chester ; and to this gentleman I am 

 indebted for the specimens of the mineral whose crystallographic rela- 

 tions I have studied. As I am informed by this careful mineralogical 

 observer, the locality is some three miles south of West Chester, where 



* This fact is not shown so forcibly by the analysis cited above as by the 

 series given by Professor Dana, on pages 310 and 311 of the fifth edition of his 

 " System of Mineralogy," to which we would refei- in illustration of this point. 



t Since the above was written, we have received from Dr. F. A. Genth his 

 very valuable paper on corundum and its associated minei-als. He regards 

 Damourite as one of the most important products of the alteration of co- 

 rundum, and gives a large number of analyses of specimens from different 

 locaUties, to which we gladly refer, as they illustrate the point made in this 

 paper, even more markedly than the analyses cited above. The Damourites 

 are evidently widely distributed minerals and characteristic features of certain 

 rocks. 



