Prof. J. P. Cooke on the Vermiculites. 257 



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 hexagonal plates, 

 which have a striation similar to that I formerly described as 

 characteristic of the Pennsylvania specimens (Amer. Journ. Sci. 

 (II.) vol. xliv. p. 201, 1867), and which is illustrated by fig. 7. 

 Professor Smith (loc. cit.) gives analyses of two varieties of ripi- 

 dolite (one compact, the other friable) as follows : — 



Compact Ripidolite from North Carolina, 



Si. M. Fe. Mg. H. 



27-00 21-60 16-63 2200 12-30=99-53. 



Friable Ripidolite from North Carolina, 



Si. M. Fe. Mg. H. 



29-15 1050 23-50 25'44 10-04=98-63. 



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 2 , and 1 1*93 H 2 O, as the mean of two 

 closely agreeing determinations. 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 out- 

 line. Some of those received from Colonel Jenks were 5 inches 

 in diameter. It has a greenish-yellow colour, 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 plates 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 parallel to this diagonal, and, like 

 the specimens from West Chester, are frequently jointed in this di- 

 rection. Like other micaceous minerals, the plates cleave readily 

 parallel to the basal plane, yielding very thin laminae, exceedingly 

 flexible, but not elastic. The optical characters are the same as 

 those of the West-Chester variety, — strong negative double re- 



Pkil. May. S. 4. Vol. 47. No. 312. April 1874. S 



