Mr.ULATCHETr's Ofyervations on Bituminous SubJIances. 137 



going pages, the fimple bituminous fubftances appear to pafs into 

 thofe which are compound ; and thefe alfo, by declining ihadcs, at 

 raft pafs into fubftances appertaining to the clafs of earths and 

 Clones. 



In the compound bituminous fubftances the prevalent earthy 

 ingredient is for the greater part generally, if not always, argillaceous; 

 and although certain calcareous grits (fuch as die Portland ftone*) 

 as well as limeftones and marbles are found impregnated with 

 bitumen, yet I know not of any inftance in which this happens to 

 the degree requifite to form a combuftible fubftance. 



This curfory view of the fimple bitumens, and of their combina- 

 tions, would be fufficient as an introduction to the principal fubject 

 of this paper ; but, to complete the feries, I (hall make fome obfer- 

 vations on the vegetable fubftances which contain bitumen, and 

 £hall afterwards mention the mixtures of bitumen with metallic 

 oxides, 



§ 4. 



WHEN we confider the facts which apparently prove that vege- 

 tables have contributed principally to the formation of bitumen 

 we have every reafon to expect that mixtures of vegetable matter 

 with bitumen fhould frequently occur. But by the mixture of 

 bitumen with the parts of vegetables, wc undcrftand the remains 

 and parts of vegetables mixed and connected with the bitumen 

 which they themfelves have produced. 



This feems to be the nature of the fubftance called 



•"The Portland ftone, when recently broken in the quarries, emits a flrong bituminous 

 odour, like the bituminous limeftone or ftink-ftone. It is alfo full of extraneous fofiils, or 

 at leaft the veftiges of them. 



Vol. IV. T BlTU- 



