NOTICES OF MEMOIRS. 
60 
It may be said, at first sight, that both the given theo^ 
ries are equally inadequate to assist the practical man r 
that, according to either, bituminous substances may be 
found impregnating the earth in any formation. But re- 
flection will show that if the geologist makes himself 
thoroughly acquainted with the structure of a district he 
'will probably acquire some means of judging as to the pro- 
bability of the occurrence of hydro-carbons therein. 
In the May number of the “ Geological Magazine ” Mr. 
George P. Wall F.G.S. replies to the remarks on his 
views contained in the review just noticed. Mr. Wall 
details briefly the proofs on which the origin of bitumen 
by direct conversion from wood is founded, and shows 
from geological evidence thn probability of this theory, and 
, the improbability of the view that the bitumen is derived 
from pre-existing carbonized vegetables. 
The mineralization of vegetable substances, whether 
tending to the production of coal and lignite or bitumens, is 
accompanied by the production of gases and especially of 
hydro-carbons. Over the whole bituminous area in Tri- 
nidad and Venezuela, wherever the liquid petroleums are 
issuing from the surface they are accompanied with an 
emission of gas more or less inflammable and frequently 
associated with water and mud. It is evident that the 
pressure of the gases generated in the production of the 
bitumens is the active agent in the delivery of the oil and 
water, which arrive at the surface perfectly cooL These 
oils and pitchy substances rapidly lose their volatile and 
solvent principles and consolidate ; where exposed to the 
solar action a further evaporation takes place and a hard 
brownish-black bitumen remains, possessing a consider- 
able proportion of earthy impurity. Wherever the surface 
is favorable for the accumulation of tin pitchy discharges. 
