Chap. XII.] 
OKIGIN OF PETKOLEUM. 
307 
Fishes of the period. Thus, in the Caithness Flags of that epoch, we 
know perfectly that the beds have been rendered bituminous by their 
numerous Ichthyolites (see p. 258, ante). 
Ascending from the Old or great Coal-period to the youngest Tertiary 
formations, we are seldom at a loss to account for the presence of bitumen 
and petroleum ; for in most of these deposits both animal and vegetable 
substances abound. Thirty-eight years have elapsed since I described the 
process of distilling oil by the application of heat to rocks of the age of the 
Lias in the Tyrolese Alps, which had been rendered highly bituminous by 
the immense quantity of fossil Fishes entombed in them *. Indeed as 
we ascend through the series of Secondary deposits, highly bituminous 
shales are sometimes found, as in the well-known case of the Kimmeridge 
Clay. Again, the large pitch-lakes of Trinidad and Venezuela are known 
to exude from Tertiary lignites, such as also yield bitumen in Persia and 
other places, especially in the great tract on the south-west shores of the 
Caspian, extending inland from Baku. 
Although I cannot yet accept as widely applicable the theory of my 
distinguished friend, M. Abich, that, in other cases, petroleum is a com- 
pound primitive body engendered in the interior of the globe (see note, 
p. 27, ante), the recent researches of that eminent French chemist Berthelot 
have gone far to sustain that view, and to show that this substance may 
have a purely mineral as well as an animal and vegetable origin. Adopting 
the hypothesis suggested by Daubree, that the terrestrial mass contains 
free alkaline metals in its interior (apparently a part of the volcanic 
theory of Davy, as supported by Daubeny), M. Berthelot, believing that 
carbonic acid does infiltrate through various parts of the crust, simply 
asks us to admit that this acid may descend to the heated alkaline metals ; 
and then the result must be the formation of acetalides. After various 
chemical reactions which he explains, these substances would, under given 
conditions, constitute petroleum. M. Berthelot sums up in these em- 
phatic words : — " "We can thus conceive the production, by a purely mine- 
ral method, of all the natural hydrocarbons, — the intervention of heat, 
water, and alkaline metals. Lastly, the tendency of the carbides to unite 
together so as to form matters more condensed suffices to account for 
the formation of these curious compounds. This formation can also be 
effected in a continuous manner, since the reactions which produce it are 
incessantly renewed" f . 
The consideration, however, of the origin of petroleum or stone-oil will 
be resumed in the 18th Chapter, when the able views of Dr. Sterry Hunt 
on this subject will be explained. It will then appear, by reference to the 
Reports on the Geology of Canada, that in North America petroleum is in- 
digenous, on a very large scale, to the Lower Silurian, Lower Devonian, 
and Lower Carboniferous limestones. 
Annals Philos. 1828. 
t Annales de Chimie, Dec. 1866, vol. ix. p. 482, 
x 2 
