179° 



12 HlSfORr of the SOCIE'tr. 



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 mod 

 common kinds of granite will be produced, confiding of per- 

 fect cryftals of fchorl, mica or garnet, inclofed in a confufed 

 mafs of felt-fpar, quartz and fchorl. 



If 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 fchorl 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 fmall 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 fchorl, 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. 



Thus, all the varieties among unftratifled fubftances may be 

 accounted for by the different circumftances in which each of 

 them palled from a liquid to a folid ftate. 



Mareh'i ^ r Clerk read the fecond part of his Scheme for weighing 



Mr cierk on up Ships funk under Water. \Vide fupra, Feb. i.] 



weighing up ^ m • /■• i • r*-» /* 



top*. Mr Clerk not having finilhed 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 Tranfactions can ap- 

 pear. 



Lit* 



