120 Mr. W. J. Lewis's Crystallographic Notes. 



Lewis. Koch. 



frt m 



al 



ae 



e c 



Calcu- 



Observed 



Calcu- 



Ob- 



lated. 



means. 



lated. 



served. 



2o 49 



*25° 49 (mean of 5 best) 

 25 45^ (mean of 20) 



25 5i 



26 3i 



44 3k 





44 6 



44 6 



69 H 



69 2 (mean of 5) 



68 50 



68 56 



41 57 



*41 57 (mean of 6) 



42 20 



41 19 



71 12 



71 7^ (mean of 6) 







LOS 48 



108 42 







me 



me, 



The development requires ns, in my opinion, to take m as 

 (110). The plane e might conveniently have the indices 

 (1 3); and the elements would then be: — 



System rhombic. 

 D=(0 1 0, 1 1) = 29° 5'-5; E = (00 1, 1 l) = 48 c 59'-6; 



F = (100, 110) = 25°49 / . 

 or a : 6: 0=1 : 2-067 : M50. 



The mineral is specially interesting, as, from Dr. Koch's 

 analysis, it seems to be a compound of ferric oxide and titanic 

 acid, and to be therefore a dimorphous form of ilmenite. Its 

 crystal-elements do not approach those of Brookite sufficiently 

 near to justify us in considering it isomorphous with the latter 

 mineral. It offers, therefore, a fresh instance of the peculiar 

 connexion which exists between oxide of iron and oxide of 

 titanium. 



Ludlamite. — In the Brooke collection is a specimen with a 

 label in Heuland's handwriting, " Phosphate of Iron on a fossil, 

 Stosgen near Linz on the Rhine, new" The phosphate of iron 

 is in minute pale green translucent crystals, having the charac- 

 teristic three-faced-wedge habit of ludlamite on the free termi- 

 nations. The best crystals of Cornish ludlamite, as described 

 by Prof. Maskelyne, give angles which vary greatly, owing 

 to more or less parallel growth. The crystals from Stosgen, 

 although half a dozen different crystals have been tried, have 

 such imperfect faces that no measurements have been yet ob- 

 tained which render a comparison with the ludlamite from 

 Cornwall possible. They seem to have a good cleavage, 

 which, however, manifests the irregular growth of the minute 

 crystals by the indefiniteness of the reflexion obtained from 

 the cleavage-face. They are deposited on the sides of a cavity 

 in the midst of a small mass of greenish-grey matrix, which 

 consists of bundles of coralloid structure containing appa- 

 rently a quantity of the same substance. I hope during the 



