284 ANNUAL OF SCIENTIFIC DISCOVERY. 



to the air become less fluid, and, partially by evaporation, partly by 

 oxidation from the air, eventually become solid and changed into 

 mineral pitch. These substances, which are doubtless of organic ori- 

 gin, occur in rocks of all ages, from the Lower Silurian to the Ter- 

 tiary period inclusive, and are generally found impregnating lime- 

 stones, and more rarely sandstones and shales. Their presence in 

 the lower palaeozoic rocks, which contain no traces of land plants, 

 shows that they have not in all cases been derived from terrestrial 

 vegetation, but may have been formed from marine plants and ani- 

 mals ; the latter is not surprising when we consider that a considera- 

 ble portion of the tissues of the lower marine animals is destitute of 

 nitrogen, and very similar in chemical composition to the woody fibre 

 of plants. Besides the rocks which contain true bitumen, we have 

 what are called bituminous shales, which, when heated, burn with 

 flame, and by distillation at a high temperature yield, besides inflam- 

 mable gases, a portion of oil not unlike in its characters to petroleum. 

 These are in fact argillaceous rocks intermixed with a portion of or- 

 ganic matter allied to peat or lignite, which by heat is decomposed, 

 and gives rise to oily hydrocarbons. These inflammable or lignitic 

 shales, which may be conveniently distinguished by the name of py- 

 roschists, are to be carefully distinguished from rocks containing 

 ready-formed bitumen ; this, being easily soluble in benzole or sul- 

 phide of carbon, can be readily dissolved from the rocks in which it 

 occurs, while the pyroschists in question yield, like coal and lignite, 

 little or nothing to these liquids. 



It is the more necessary to insist upon the distinction between lig- 

 nitic and bituminous rocks, inasmuch as some have been disposed to 

 regard the former as the source of the bitumen found in nature, which 



^j * 



they conceive to have originated from a slow distillation of these mat- 

 ters. The result of a careful examination 'of the question has, how- 

 ever, led us to the conclusion that the formation of the one excludes 

 more or less completely that of the other, and that bitumen has been 

 generated under conditions different from those which have trans- 

 formed organic matters into coal and lignite, and probably in deep 

 water deposits, from which atmospheric oxygen was excluded. Thus 

 in the palaeozoic strata of North America we find in the Utica and 

 Hamilton formations highly inflammable pyroschists, which contain 

 no soluble bitumen, and the same is true, to a certain extent, of some 

 limestones, while the Trenton and Corniferous limestones of the same 

 series are impregnated with petroleum or mineral pitch, and, as we 

 shall show, give rise to petroleum springs. The fact that intermediate 

 porous strata of similar mineral characters are destitute of bitumen 

 shows that this material cannot have been derived from overlying or 

 underlying beds, but has been generated by 'the transformation of or- 

 ganic matters in the strata in which it is met with. This conclusion 

 is in accordance with that arrived at by Mr. Wall in his recent inves- 

 tigations in Trinidad. He has shown that the asphalt of that island 

 and of Venezuela belongs to strata of the tertiary formation (of up- 

 per miocene or lower pliocene age), which consists of limestones, sand- 

 stones, and shales, associated with beds of lignite. The bitumen is 

 found not only in the famous pitch lake but in situ, where it is con- 

 fined to particular strata which were originally shales containing veg- 



