490 NOTE ON THE ORIGIN OF " KEROSENE SHALE," 



does not remove it, as the obvious question at once suggests itself, 

 What caused the oil-wells? and of this no satisfactory account 

 can be given. 



It might be argued that such oil-wells might have had an origin 

 similiar to those of Trinidad, which are associated with the 

 famous Asphaltum Lake of La Brea. At Trinidad, however, the 

 oil and asphaltum is considered by Messrs. S. P. Wall and J. G. 

 Sawkins, F.G.S.,"^ to have originated from the alteration in situ 

 of ligneous deposits, together with, perhaps, a slight admixture of 

 animal material, especially shells. They consider the Asphaltum 

 Lake of La Brea, which has a superficial area of 99J acres, and 

 is estimated to contain about 3,168,000 tons of bitumen (the 

 average depth being supposed to be 20 feet)> to have resulted 

 simply from a segregation and concentration of the bitumen in 

 local depressions in the asphaltic sands and shales. 



The same authors state {loc. cit. p. 144) : — "The conversion 

 from the ligneous into the bitumineous structure may be seen in 

 any stage, from the first deposit (usually parallel with the fibres) 

 to the total obliteration of organic texture, when nothing but the 

 external form of the wood reoiains." They conclude that asphaltum 

 has been formed from vegetable material by direct conversion at 

 the ordinary temperature. The same stratum may be lignitic at 

 one point, and asphaltic at another, the difference being attributed 

 to the different chemical reactions which have taken place, the 

 tendency being for lignite to be formed, where the deposit is of a 

 pure carbonaceous nature, and asphaltum, where a large propor- 

 tion of earthy matter exists. The first stage in the process of con- 

 version of woody matter into asphaltum consists in the formation 

 of asphaltic oil ; this oil rises in springs from comparatively shallow 

 depths. The residue after the separation of this oil is usually the 

 ordinary asphalt. 



* " Memoirs of the Geological Survey of the West Indies." Part I. 

 " Report on the Geology of Trinidad." By S. P. Wall and J. G. Sawkins, 

 F.G.S., 1860, pp. 143-147. 



