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



cause they illustrate the characteristics of basic water, which will 

 be contrasted with those of water of crystallization in our de- 

 scription of the following 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 vari- 

 able, 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 Newhaven, who named it 

 after W. W. Jefferis, Esq., of West Chester; and to this gentle- 

 man I am indebted for the specimens of the mineral whose crys- 

 tallographic relations I have studied. 



The crystals of Jefferisite cleave like mica, affording thin but 

 unelastic laminae. The cleavage-planes are marked triangularly 

 by lines crossing at angles of 60° and 120°. In some cases 

 there is a jointing as in crystals of mica, parallel to the shorter 

 diagonal of the rhomb. One crystal sent me by Mr. Jefferis is 

 the half of a rough hexagonal prism 1 J inch high by 2 inches 

 in diameter. The plane of the optical axes, as in the larger 

 number of micas, is parallel to one of these lines, as indicated 

 in Plate III. fig. 6, coinciding with the shorter diagonal of the 

 rhombic prism, which appears to be the fundamental form in all 

 this class of minerals, and from which the hexagonal form is de- 

 rived by the truncation of the two acute angles. The double 

 refraction is strongly negative ; but the angle between the optical 

 axes varies in the most remarkable manner. I have measured 

 angles on different plates, of 27°, 24°, and 10°, and observed 

 many intermediate conditions. Owing to the deep yellow colour, 



* 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 edi- 

 tion of his ' System of Mineralogy/ to which we would refer in illustration 

 of this point. 



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

 his very valuable paper on corundum and its associated minerals. He 

 regards Damourite as one of the most important products of the alteration 

 of corundum, and gives a large number of analyses of specimens from dif- 

 ferent localities, 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. 



i 



