1 88 Chemistry. [Lecture 30. 



this have derived their names, as carnelion, which 

 is of a flesh colour. In others, the colouring 

 matter is disposed in a fantastical or irregular 

 manner, as in the mocao stone. Some have 

 transverse plates, as the onyx. 



Another kind of rock, which is chiefly com- 

 posed of flinty matter, is that of porphyry, in 

 which crystals of felspar are imbedded in masses 

 of the same substance. 



There are several other kinds of rocky strata, 

 which, though not entirely composed of flint, 

 have yet a considerable quantity of it; as the 

 granite and common whin. 



The granite is composed of quartz, felspar, 

 and mica, diifering considerably in colour ac- 

 cording to the admixture, but is always composed 

 of hard flinty grains cohering together. This 

 was formerly famous in Egypt; some of the 

 monuments, which were afterwards transported 

 to Rome, were made of it. These are all va- 

 rieties of the same stone, which is become the 

 object of more general attention since its use in 

 the manufacture of porcelain. 



The common whin is \vell known ; but a par- 

 ticular kind, basaltes, is remarkable for being 

 crystallized in large columns, such as the Giant's 

 Causeway, where the pillars are nearly of a re- 

 gular figure, plainly resembling each other in 

 size, and in the number of their sides, most of 

 them hexagonal, and in close contact with each 

 other. This particular arrangement seems to 



