Cryjlallizations obferved on Glajs. 541 
are really affemblages of plates or tables, difpofed in 
every direction round a common axis. 
Laftly, The hone on which the columns of bafaltes 
generally refts, and which fometimes alfo is fupported 
by thefe columns, being of the fame nature and texture 
as the columns themfelves, teems to be a mafs irregu- 
larly cryftallized, analogous to the irregularly-fhaped 
malfes in the Specimens of glafs N° 1. and 2. which 
evidently con flit of a fimilar fubftance as the neighbour- 
ing cryftals, and leem to have been compofed of a num- 
ber of thefe cryftals indiftinAly united; for the peculiar 
figures of cryftals are dillinct only when they are inhi- 
bited, or when they are feparated from each other by a 
pellucid or differently-coloured medium. A medium of 
this kind appears between the vitreous cryftals, and is 
nothing elfe than the more fluid paits of the glais, 
which longer refill the concretion, but which, by a fur- 
ther continuance of the heat, would have become, with 
the parts already cryftallized, one uniform, wdiite, opaque 
fubftance, without any interposition of tranfparent glafs, 
or diftinaion of cryftal, except on the furface, as in fpe- 
cimen N° 5, in which the cryftals ftand prominent from 
the indiftina mafs, and unenveloped in any medium, in 
the fame manner as the bafaltic cryftallizations appeal 
Handing above the mafs of ftone 01 Ictvci w hich fuppoits 
them. 
Further obfervations on the bafaltic and vitreous cry- 
ftals may probably luggeft more inftances of analogy 
4 B 2 between 
