1873.] 369 



[Genth. 



reduced to an impalpable powder, with tolerable ease. "We have there- 

 fore no means to determine the quantity of corundum, which is mechani- 

 cally mixed with another mineral. This is unfortunate, as it will leave 

 us in doubt about the true constitution of some varieties of lesleyite.and 

 of ephesite. 



Magnesia was always precipitated by ammoniac phosphate. Of the 

 alcalies the potassic sodic and lithic chlorides were weighed together, 

 then tbe potassium were separated and weighed as potassic platino- 

 chloride, and in the filtrate, after the removal of the excess of the plati- 

 num and the conversion of the alcalies into sulphates, the quantities of 

 sodium and lithium were determined by indirect analysis. As however, 

 in many cases the quantity of lithia, indicated by the spectroscope, was 

 only small, very few such determinations have been made. 



These being the more important points with reference to the modes of 

 analysis, I shall now proceed with the description of the minerals. 



1. COKUNDUM. 



As it would require unnecessary repetition, I shall not at present 

 describe the numerous varities of corundum, but shall mention their 

 peculiarities, when treating of the minerals associated with them, and of 

 their relation to each other. 



2. Spinel. 



Corundum altered into spinel occurs at several localities. 



a. The most interesting variety comes from Hindostan, and was found 

 in a lot of corundum crystals, purchased by Messrs. S. S. White <k Co., 

 to whom I am indebted for specimens. 



The surface of these crystals or crystalline masses is rough, and to 

 many of them reddish orthoclase and a dark mica are attached, showing 

 their matrix to have been granite. Some pieces of the corundum contain 

 also small scales of reddish-white margarite. The crystals are too indis- 

 tinct for measurement, but the basal plane and one or two pyramids can 

 be observed. Many are completely altered, most of them, however, show 

 that the alteration began at the surface and has irregularly penetrated 

 the crystals towards the centre, often leaving a nucleus of brownish-gray 

 cleavable corundum, or patches of the same irregularly distributed 

 through the mass. Most of the crystals are from a half an inch to about 

 two inches in size, rarely larger. 



The spinel, which is the result of this alteration, has a black color, 

 granular crystalline structure and a submetallic to vitreous lustre. Its 

 powder is gray ; it is slightly magnetic— H.=S. Sp. Gr.=4.208. 



b. In the shaft on the road from Unionville to Kennett Square, associ- 

 ated with granular grayish-white talc and greenish actinolite occurs in- 

 timately mixed with chlorite, a black granular mineral of vitreous lustre 

 and a hardness superior to that of quartz. It was impossible to obtain 

 it for analysis in a state of purity, but sufficiently so to prove it to be 

 spinel. 



c. The vein of spinel passing through the chlorite at the Culsagee Mine, 



A. P. S. — VOL. XIII. 2u 



