Chap. XVIII.] OIL-SPRINGS OF NORTH AMERICA. 
445 
States two other salt-formations are met with, the one above and the other 
below the Carboniferous Limestone. These salt-formations, derived probably 
from the drying up of sea-basins or salt lakes, are naturally limited in their area 
when compared with the widespread oil-bearing formations which were depo- 
sited in the open sea. 
With regard to the origin of petroleum, Dr. Hunt adopts the conclusion de- 
duced by Mr. "Wall from his researches in Trinidad *, that its source is to be 
found in a peculiar transformation of organic matters, either animal or vegetable, 
under conditions unlike those which give rise to coal. This process, according 
to Dr. Hunt, has probably been effected in comparatively deep waters, where 
oxygen was excluded, and the carbon, retaining its maximum of hydrogen, was 
converted into liquid or solid bitumen, which is thus in these cases the only 
representative of the organic tissues which in different conditions would have 
given rise to coaly or lignitic matters. The preservation of these tissues in such 
forms is thus incompatible with the production of petroleum j and there exists 
no connexion whatever between this substance and the beds of flaming coal and 
schists (pyroschists) which, Dr. Hunt adds, are incorrectly termed bituminous, 
not because they contain bitumen, but because they may be made to yield 
volatile hydrocarbons by destructive distillation. 
* See Eeport on the Geology of Trinidad, Mem. Geol. Survey, 1860 ; also Quart. Journ. Geol. Soc. 
vol. xvi. p. 467. 
