c 6 Dr. Beddoes’s Objervations on the 
mieu, “ I have obferved the tranfition from fhoerl in a 
“ mafs nearly homogeneous (I fay, nearly homogeneous, be- 
“ caufe 1 know of no ftones, belonging, as thefe do, to the 
“ primitive mountains, without indications of a reparation of 
a feveral fubftances which were incorporated together in a 
« pafte, or rather which are generated in that pafte) to black 
“ and white granites, with large grains, and compofed of 
c i ne avly equal quantities of white feldfpath and black fhoerl. 
“ The tranfition depends altogether on an increafed proportion 
“ of feldfpath and on the enlargement of its grains ; a 
“ phenomenon which leaves no room to doubt, that all thefe 
“ ftones belong to the fame fyftem of mountains 
By obfervations like thefe, which the fpecimens I either 
poflefs, or have examined, corroborate and compleat, I am 
perfuaded, that when once it becomes an object of attention, 
perfons who have an opportunity of exploring countries where 
bafaltes and granite abound, will eafily find a fucceffion of 
fpecimens beginning at the former and terminating at the 
latter. Nor is it perhaps difficult to affign highly probable 
reafons, why a mixture of different earths with more or lefs 
of metallic matter, in returning from a {fate of fufion to a 
fclid confiftence, may affume fometimes the homogeneous 
bcj'altic, and fometimes the heterogeneous granitic internal 
ftruflure. No faff is more familiar than that it depends alto- 
gether ori the management of the fire, and the time of cool- 
ing, whether a mafs fhall have the uniform vitreous fradure, 
or an earthy broken grain, arifing from a confufed cryftalliza- 
tion. The art of making Reaumur’s porcelain confifts en- 
* Journal de Phyfique, September 1790, p. 196. X have feveral fpecimens 
from the whin rocks about Edinburgh perfeaiy anfwering to this defcription, 
and fo like fome Vefuvian lavas that it is impolfible to diftinguifli them. 
i tirely 
