398 JOSEPH STRUTHERS 



deposit, up to 20 per cent. The grain of asphaltic limestone 

 is extremely fine, and under the microscope each grain ap- 

 pears completely coated with asphaltum. It is this prop- 

 erty which renders the natural product of greater value than 

 an artificial mixture of asphaltum, fine sand, and pulverized 

 limestone. 



The formation of natural asphaltum is attributed by 

 some geologists to the slow decay and decomposition of veget- 

 able matter whereby the contained hydrocarbons have been 

 distilled and subsequently condensed in the adjoining rocks, 

 the product thus collected forming asphaltum deposits. The 

 formation of artificial asphaltum in the manufacture of gas 

 by the distillation of coal in a retort is quoted as proof of this 

 theory. In this process a considerable portion of tarry mat- 

 ter passes over and is collected during the distillation which 

 yields, by a second distillation, naphtha and other volatile 

 products, leaving as a residue in the retort a tarry substance 

 possessing the properties of asphalt. 



Other geologists claim that asphaltum is the result of the 

 decomposition of certain petroleums under the influence of 

 heat, accompanied by polymeric changes not fully understood, 

 whereby the more volatile constituents of the oil are expelled, 

 leaving the residue in the form of solid asphaltum. 



Asphaltum is seldom found native or pure, the principal 

 known deposits containing it in admixture with other materi- 

 als, chiefly earthy matter, sand, or sandstone and limestone. 

 It occurs disseminated throughout many crystalline rocks, 

 and is not restricted to any one geologic formation. It is not 

 known, however, in the Archaean age. In many cases it has 

 been deprived of volatile matter, which has caused it to re- 

 semble anthracite. It occurs also in veins that have been 

 evidently injected into fissures, while in a plastic condition, 

 exemplified by the minerals albertite and grahamite. It 

 exists as a Hquid in bituminous hmestones and sandstones, 

 from which it oozes, constituting the so called tar springs. 



The most important of the Hquid asphalt deposits are 

 found in southern California, of which the Las Conchas mine 

 on the ocean near Carpentaria has been operated successfully. 



