222 J, P. Cooke, Jr., on Cryophyllite. 
although the infusible mica corresponding to cryophyllite has 
not yet been observed. It is manifest that the theory here ad- 
vanced would be very greatly substantiated if it could be shown 
that cryophyllite is actually associated with a distinct but 1s0- 
morphous miea having the type ratio 1:1, and that it does ac- 
tually influence the composition of its associate in the way we 
ate indicated. Such is the case, as we believe, at the Rockport 
ity. 
Lepidomelane,—The mica which is associated with cryophyllite 
at Rockport (or as we should rather say, with which eryophyl- 
lite is associated, for cryophyllite is the subordinate species) 1S 
an iron mica of the species lepidomelane. This is the common 
mica of the great granite ledges which form the extremity of 
Cape Ann. In the granite itself, however, it occurs only In 
small flakes forming a very small proportion of the whole mass; 
but in the numerous veins which intersect the rock, lepidome- 
lane is found in crystals of considerable size and sometimes 10 
plates several inches in diameter. The vein, from which most 
fishoot of one : 
