NOMB XXI. BITUMINOUS ROCKS. 147 



NOME XXI. BITUMINOUS ROCKS. 



The chief bituminous substances are naphtha, 

 or pure rock oil, as fluid and* transparent as 

 water ; petrol, which is less fluid and pure, when 

 it is yet more impure it becomes mineral tar. 

 Of mineral pitch there are three diversities : 

 Maltha, of a brownish colour and earthy con- 

 struction 3 Asphalt, pure and black; and the 

 elastic, or mineral Caoutchou. 



All the bitumens belong more strictly to the 

 province of chemists, who now arrange them 

 after the vegetable substances, from which, like 

 coal, they all seem to be derived. 



They are most commonly found in the proxi- 

 jnity of that mineral, and in its most usual at- 

 tendant rocks, limestone and sandstone. In 

 Siberia, bitumen has even been observed in balls 

 of chalcedony. It sometimes also appears in 

 veins, that traverse that argillaceous glutenite 

 called grauwack ; and in veins of calcareous 

 spar in basalton, or the transitive grunstein of 

 Werner. The asphalt occurs in mineral veins, 

 like the caoutchou. The chief bituminous rocks, 

 however, are limestone and sandstone; the for- 



