GEOLOGICAL COLLECTION. 27 1 



In the first class we find the earthy and saline 

 substances: as, 1st. The pegmatite, which, under 

 the vulgar name of petunzd, furnishes the glazing 

 matter for chinaware, and the detritus of which 

 produces the earth called kaolin, of which that 

 valuable sort of ware is made. 2d. Common 

 porphyry. 3d. The syenite, which forms the 

 rocks of the cataracts of the Nile, and from which 

 the Egyptian obelisks were hewn. 4th. The 

 common granite, which is the lowest bed of the 

 crust of the earth within our reach. 5th. The 

 phthanite, of which touchstones are made. 

 6th. The potstone, which in several countries is 

 formed into cooking and other domestic utensils. 



In the second class, that of combustible sub- 

 stances, we see various species of coal, and a spe- 

 cies of fossil wood used by jewellers under the 

 name of jet. 



In the third class, containing metallic sub- 

 stances, we find specimens of the principal me- 

 talliferous rocks, from which are extracted cop- 

 per, iron, lead, tin and zinc. 



The fourth class contains the rocks of igneous 

 origin, according to many geologists ; of aqueous, 

 according to others. These rocks offer a pro- 

 blem, for the solution of which, we may compare 

 their characters with those specimens which are 

 in the preceding and following classes. 



