Correspondence. 237 



discharge the oily fluid for two montlis.' The bitumen may be 

 shown to have originated in these strata charged with vegetable re- 

 mains, by the coast and ravine sections, where such strata are ex- 

 posed containing wood in various stages of conversion ; whilst 

 intervening beds of sand or clay, in which no vegetable matter was 

 ever contained, are entirely free from from all traces of bitumen, 

 though equally adapted for fixing that substance, if it had been pro- 

 duced by distillation and risen from below. 



It was not a part of the geological enquiry, respecting the bitu- 

 mens of Trinidad, to investigate the chemical nature of this change 

 of wood into the former; hence the term " special mineralization" 

 was proposed as a record of the fact, and to distinguish this process 

 from that which produces coal or lignite. It would seem to be due, 

 however, to a reaction of the earthy matter of the containing stratum 

 on the vegetable remains, since, where the latter predominate so as 

 to form a pm'e mass of woody matter, lignite results ; beds of which 

 also exist in the same districts, but are quite distinct from the bitu- 

 minous strata, which only occur when a large amount of earth is 

 associated with the woody substance. The forest swamp soils on 

 the coast, in progress of formation at the present day, exhibit many 

 analogies with the bituminous beds, viz., by containing up to 75 

 per cent, of vegetable debris, alternately overflowed by the sea and 

 subjected at low tide to the influence of a powerful sun, an active 

 chemical action, distinguished by the sensible production of sulphu- 

 retted hydrogen and other gases, is developed; and subsequent changes 

 of level would reduce these swamp soils to precisely the same con- 

 ditions as those presented by the bituminous strata. 



We know that the mineralization of vegetable substances, whether 

 tending to the production of coal and lignite, or bitumens, is accom- 

 panied by the production of gases and especially of hydro-carbons. 

 Over the whole bituminous area in Trinidad and Venezuela, wherever 

 the liquid petroleums are issuing from the surface, they are accom- 

 panied with an emission of gas more or less inflammable, and fre- 

 quently 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 solvent principles and consolidate ; where 

 exposed to the solar action a further evaporation takes place and a 

 hard brownish black bitumen remains, possessing a considerable 

 proportion of earthy impurity. Wherever the surface is favourable 

 for the accumulation of the pitchy discharges, the bitumens, still 

 plastic, flow together from various centres of emission and coalesce 

 into a more or less extensive aggregation of the substance. The 

 " Pitch Lake " of Trinidad is merely an instance of this coalescence 

 on a larger scale than usual. 



Many specimens of vegetables, undergoing this species of minera- 

 lization, were subjected to the late Mr. Stermann Criiger, Colonial 



' Numerous specimens illustrating the change of wood into bitumen were deposited 

 in the Mugeums of Jeroneport, and Port of Spain, Trinidad. 



