Prof. J. P. Cooke on the Vermicuiites. 265 



Hallite occurs in large rough six-sided prisms, with easy mi- 

 caceous cleavage. There are two varieties, differing markedly in 

 colour — green and yellow • and I am indebted to Mr. Hall for 

 the following facts in regard to the locality and associations of 

 this species. Mr. Hall writes : — " The mineral is found at East 

 Nottingham, in the serpentine formation of south-eastern Penn- 

 sylvania, three miles south of Oxford, in Chester County ; and I 

 know of do other locality. I think the green and yellow varie- 

 ties are very closely related, and may possibly pass from one into 

 the other; but I have no positive proof that they do. The 

 crystals are found in nests or pockets ; and the two colours are 

 not found in the same nests. The green crystals are imbedded 

 in a steatite earth or base of the same colour as the crystals, and 

 the yellow in a yellow earth ; and sometimes nests containing 

 the opposite varieties are only a few feet apart in seams of the 

 serpentine rock." 



As the following analyses show, the two varieties have essen- 

 tially the same composition ; and the only difference that could 

 be detected was in the degree of oxidation of the iron. The 

 yellow crystals appeared to be more weathered than the green ; 

 and on the last the green colour frequently fades out towards 

 the centre of crystals, thus giving indications of a metamor- 

 phosis by which one variety may pass into the other. 



Under the microscope these scales of the mineral show a re- 

 markable appearance. Between the greenish or nearly colour- 

 less plates are seen elongated scales of a yellow mineral resem- 

 bling closely in colour thin scales of Jefferisite. They are more 

 or less spear-shaped in form, although usually very narrow, and 

 lie accurately in parallel lines, which cross at angles of 60° and 

 120°, like the magnetic oxide of iron in the Muscovite from 

 Pennsbury, Pa., or the microscopic crystals in the Biotite of 

 South Burgess of Canada ; and the phenomenon of asterism, seen 

 so beautiful with the plates of the last, can also be seen with thin 

 laminae of Hallite. It was impossible to free the mineral from 

 this admixture ; but specimens were selected for analysis as free 

 from it as possible. It was also impossible to determine its exact 

 nature. The scales had not a definite form ; but there was a 

 tendency to a rhombic shape, which is well described by the 

 term "spear-shaped;" and though the material is so widely 

 distributed through the crystal, the total mass must be very small. 



This mineral is not so hygroscopic as Jefferisite, and no diffi- 

 culty was found in drying the material for analysis. When 

 ignited, it exfoliates like other species of vermiculite, but not 

 nearly to so great an extent as Jefferisite. After ignition it is 

 decomposed by hydrochloric acid. The specific gravity of the 

 green variety, mean of four determinations, 2*398 ; that of the 



