50 PROCEEDINGS OF THE AMERICAN ACADEMY 



vermiculite, and the scales of these minerals not unfrequently penetrated 

 the corundum crystals in such a way as to indicate that the corundum 



had crystallized around them. The ripidolite is 

 firm in texture, and closely resembles the variety 

 from West Chester. It occurs in hexasronal 

 phites, which have a striation similar to that I for- 

 merly described as characteristic of the Pennsyl- 

 vania specimens (Am. Jour. Sci., II. xliv. 201, 

 1867), and which is illustrated by Fig. 2. 

 Professor Smith, he. cit, gives analyses of 

 two varieties of ripidolite — one compact, the 

 other friable — as follows : — 



Compact Ripidolite from North Carolina. 



I c 



A determination of the silica in the compact mineral made by Mr. F. 

 Gooch, student in the Laboratory of Harvard College, gave 27.25 per 

 cent of SIO.,, and 11.93 N^O, as a mean of two closely agreeing deter- 

 minations. But in the specimens sent me by Colonel Jenks I did not 

 recognize the friable variety. 



The vermiculite occurs in close contact with the ripidolite, and is 

 frequently interlaminated with it, but the two are always perfectly 

 distinct, thus entirely disproving the theory that the vermiculite was 

 derived from the ripidolite by weathering. It occurs in large plates 

 having more or less of an hexagonal outline. Some of those received 

 from Colonel Jenks were five inches in diameter. It has a greenish- 

 yellow color, which is very much lighter than that of the West Chester 

 variety. The plates are strongly marked by lines crossing at angles 

 of 60° and 120°, like those from West Chester; but these lines are 

 more marked in the North Carolina variety. This variety is also much 

 more friable than the other, and readily breaks in directions parallel to 

 these lines, — yielding rhombic jilates with angles of 120° and 60°, and 

 more readily hexagonal or triangular plates, produced by the truncation 

 of the 60° angle of the rhombic plate, on a line parallel to the shorter 

 diagonal of the fundamental rhomb. The plates most readily break 



