414 



GREAT PITCH LAKE 



a- 



V 



[Ch. XVII. 



animal remains ; for portions, says Mr. Hunt, of the tissues of 

 various marine animals of low grade are destitute of nitro- 

 gen, and very similar in mineral composition to the woody 

 fibre of plants. The probability, in some cases, of an animal 

 origin has been especially inferred from the frequency of what 

 is called anthracite in the ' calciferous beds ' of the Lower 

 Silurian of New York. The anthracite of this ancient rock, 

 says Mr. Hunt, is an inspissated mineral oil. Petroleum is 

 found in formations of every age, from the Lower Silurian up 

 to the Tertiary ; but a large number of the oil-wells of the 

 United States, which have lately attracted so much attention, 

 are in Devonian rocks. 



In some instances the petroleum appears to filter slowly 

 into the wells from the porous strata around, which are satu- 

 rated with it, while, at other times, the boring instrument 

 seems to strike upon a fissure communicating with a reser- 

 voir which furnishes at once great volumes of oil.* 



The great pitch lake of Trinidad is situated, according to 



Mr. Wall, in Tertiary strata, chiefly Upper Miocene, but 

 partly, perhaps, Lower Pliocene. The asphalt is derived from 

 bituminous shales, containing vegetable remains, which are 

 sometimes seen in the process of transformation, with their 

 organic structure more or less obliterated. Occasionally the 

 bituminous substance becomes plastic and even oily, and rises 

 to the surface. f Such changes from oil to a pitch may be 

 brought about, says Mr. T. S. Hunt, partly by the evaporation 

 of the volatile ingredients, and partly by oxidation from the 

 air. 



Captain Mallet observes that, near Cape La Braye, in the 

 island of Trinidad, fluid bitumen sometimes oozes out from 

 the bottom of the sea, and rises to the surface. The same 

 author quotes Gumilla, as stating, in his 'Description of the 

 Orinoco/ that ' about seventy years ago, a spot of land on the 

 western coast of Trinidad, near half-way between the capital 

 and an Indian village, sank suddenly, and was immediately 

 replaced by a small lake of pitch, to the great terror of the 

 inhabitants. 9 ^; 



* Sterry Hunt, Canadian Naturalist, xvi. p. 468. 1860. 

 vol. vi. p. 245. August 1861. j Mallet, cited by Dr. Nugent, Geol. 



t Wall. Quart, Geol. Journ. vol. Trans, vol. i. p. 69. 1811. 



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