Correspondence. 239 



cation ; but since " E. C. H. D." hazards the observation that " bi- 

 tumen or petroleum having in some instances arisen from a ' special 

 mineralization ' of animal remain s, is a doctrine by no means gene- 

 rally accepted ; " to adduce only one of many well known instances 

 tending to confirm the truth of this view, also, viz., that of the 

 mountain limestone of the adjacent district (Derbyshire), which al- 

 most wherever highly fossiliferous contains bitumen in cracks and 

 joints of the rock, evidently derived from the animal substance of the 

 fossil remains, and not due to the influence of heat. 



In conclusion it only remains to ask, what necessity for the mys- 

 tery and difficulty with which the subject is involved in " E. C. H. D.'s" 

 estimation? To our mind the generation of bitumens is easily and 

 simply explained by the operation, at the ordinary terrestrial tem- 

 peratures, of chemical laws of which we have cognizance, and the 

 phenomena attending the emission of the oils and associated gases, 

 are equally explicable by known physical laws. The facts and cir- 

 cumstances connected with the occurrence of petroleum and bitumen, 

 which induce " E. C. H. D." to accept a " distillation theory," have a 

 directly contrary influence on us, and convince us that these sub- 

 stances are the result of a law of mineralization operating as regu- 

 larly, and almost as extensively, as that which produces coal and 

 lignite. Yours respectfully, Geo. P. Wall. 



Sheffield, March 26, 1866. 



To the Editor of the Gteological Magazine. 



Dear Sir, — Since I sent you the last communication on the junc- 

 tion of the Chalk and Thanet Sands,^ Mr. T. McKenny Hughes has read 

 a paper before the Geological Society on the same subject.^ His 

 theory of the formation of the green-coated flints is that, they are 

 the result of the solution of the chalk by carbonated water aftar the 

 deposition of the Thanet Sands ; the objection to this appears to me, 

 that by such means we should not have that remarkably even surface 

 presented by the Chalk at its junction with the Thanet Sand, nor 

 that marked peculiarity of the green-coated flint, resting on other 

 flints not presenting this peculiarity, moreover where the chalk is 

 worn by the action of carbonated water as in the pipes and furrows 

 of the chalk, we find a most uneven surface presenting no appearance 

 like that of the junction of Chalk with the Thanet Sands. The 

 tabular flint, immediately below the green-coated flints, is by no 

 means so continuous as to present an obstacle to the passage of 

 water, sufficient to account for the even imdissolved chalk, and 

 wherever pipes do occur beneath the Thanet Sands (which, as far as 

 I can ascertain, is only the case where the latter is near the surface), 

 the green-coated flints sink down into the pipe with the tabular un- 

 coated flint, which is always to be distinguished from the former. 



I believe the tabular flint has been formed subsequently to the 

 Thanet Sands, and the green-coated flint resting upon it. 



Yours, etc., George Dowker. 



Stotjkmouth, April 18tli, 1866. 



1 See ante, p. 210. 2 See ante, p. 223. 



