1790. 
March . 
Mr Clerk on 
weighing up 
hips, 
a HISTORY of the SOCIETY. 
takes place, when the cryftals of the various fubftances will in- 
termix and confound the regularity of form which each would 
have aflumed if left to itfelf. In this manner, one of the moft 
common kinds of granite will be produced, confifting of per- 
fect cryftals of fchorl, mica or garnet, inclofed in a confufed 
mafs of felt-fpar, quartz and fchorl. 
Ir the firft ftage of cooling is performed in the bowels of the 
earth, and if the folution, while ftill liquid, is by fome effort 
forced upwards, and erupted into the open air in the form of a 
lava, which being fpread thin upon the furface, and expofed to 
the air, would lofe its heat fuddenly, the cryftals of {chorl and 
of mica, originally held by the heat of the menftruum, will be 
of a large fize, having been produced in the liquid when in a 
great mafs, and when its heat of courfe efcaped very flowly, 
there will be embodied in a mafs formed of very {mall cryftals, 
fince they have been formed with great rapidity. This in fact 
is the defcription of one of the moft common lavas, which con- 
fifts of large and perfect cryftals of {chorl, embodied in a mafs 
whofe fracture is dull and rough, and which, when examined 
with a microfcope, is found to confift of a congeries of minute 
cryftals. 
Tuus, all the varieties among unftratified fubftances may be 
accounted for by the different circumftances in which each of 
them pafled from a liquid to a folid ftate. 
Mr Crerk read the fecond part of his Scheme for weighing 
up Ships funk under Water. [Vide fupra, Feb. 1.] 
Mr Crerk not having finifhed the drawings neceflary for the 
illuftration of this paper, did not put it into the hands of the 
Committee for publication ; which, however, he has undertaken 
to do, before another volume of thefe Tranfa¢tions can ap- 
pear. 
Lit. 
