236 Correspondence. 



P.S. — There is here a small geological class, the members of 

 which take the " Geological Magazine," and read " The Silurian 

 System." I have just re-perused (after a lapse of many years) the 

 chapters on Drift at the close of Sir E. I. Murchison's celebrated 

 work ; and I cannot help thinking that the accurate descriptions and 

 sound generalizations with which they abound, if re-published 

 separately, would be of great service in moderating the zeal of 

 modern subaerialists. I see Sir Eoderick accounts for combes in 

 the same way that I have lately been advocating in your pages. 

 He says " These combes and valleys could have been modelled into 

 their actual forms only by the action of a large body of water 

 overspreading their entire area. . . . The nature of the excavation 

 indicates also the action of water differently propelled at different 

 times, perhaps by tidal currents, the directions of which were 

 determined by local causes." — ^D.M. 



THE ORIGIN OF BITUMEN. 

 To the Editor of the Geological Magazine. 



Sir, — ^Tour issue for March contains an article on "Petroleum and 

 Oilfields," in which my views on the generation of bituminous sub- 

 stances are alluded to and controverted ; at the same time a desire is 

 expressed for further information respecting the occurrence of petro- 

 leum and bitumen in Trinidad. 



If " E. C. H. D.," author of the article in question, had carefully 

 considered the evidence adduced in the "Eeport on the Geology of 

 Trinidad," he could scarcely have arrived at the conclusion that the 

 direct production of bitumen from vegetable remains is doubtful, or 

 the proofs of the production " defective," since this view is not pro- 

 posed as a theory, but stated as an evident fact, beyond the range of 

 discussion. 



To detail, briefly as possible, the proofs on which this origin of 

 bitumens is founded, viz. : — the existence over the bituminous dis- 

 tricts of strata more or less charged with vegetable debris, with the 

 woody matter in progress of conversion into bitumen, which con- 

 version is induced entirely by internal chemical action, and inde- 

 pendent of any extraneous influence, such as heat. This process is 

 distinguished by the production of a dense, very black petroleum, 

 which oozes out of the vegetable mass and only ceases to be formed 

 on the complete change of the woody substance into bitumen ; and 

 is also accompanied by the formation of hydro-carbons. This oily 

 fluid gradually solidifies (probably from the evaporation of a volatile 

 solvent), leaving a black, very pure bitumen, locally known as 

 " glance pitch." The residue of the wood is represented by a 

 brownish black bitumen of impurer nature, in which all trace of 

 vegetable structure has disappeared. The operation of this con- 

 version is so intense, that hand specimens of the wood, when isolated 

 from their earthy matrix and placed in a room, have continued to 



