278 The Origin and Relations of the Carbon Minerals. 



they escape. This is, however, not certiiin, for the gases, as we 

 find them, are always mixtures and never pure. In the liquid 

 evolved products, the petroleums, this is emphatically triie, for 

 we combine under this name fluids which vary greatly in both' 

 their physical and chemical characters ; some are light and 

 ethereal ; others are thick and tarry ; some are transparent, 

 some opaque ; some red, some brown, others green ; some have 

 an offensive and others an agreeable odor; some contain asphalt 

 in large quantity, others paraffine, etc. Thus they form a 

 heterogeneous assemblage of liquid hydro-carbons, of which naph- 

 tha and maltha may be said to form the extremes, and which 

 have little in common, except their undetinable name. The 

 causes of these differences are but imperfectly understood, but 

 we know that they are in pai't dependent on the nature of the 

 organic material that has furnished the petroleums, and in ptirt 

 upon influences affecting them after their formation. For ex- 

 ample, the oil which saturates the Niagara limestone at Chicago, 

 and which is undoubtedly indigenous in this rock, and probably 

 of animal origin, is black and thick ; that from Enniskillen, 

 Canada, is also black, has a vile odor, jirobably in virtue of sul- 

 phur compounds, and we have reason to believe is derived from 

 animal matter. The oils of northwestern Pennsylvania are 

 mostly brown, sometimes green by reflected light, and have a 

 pungent and characteristic odor. These are undoubtedly de- 

 rived from the Hamilton shales, which contain ten or twenty 

 per cent, of carbonaceous matter, apparently produced from the 

 decomposition of sea-weeds, since these are in j)laces exceed- 

 ingly abundant, and nearly all other fossils are absent. 



The oils of Italy, though varying much in appearance, have 

 usually an ethereal odor that is rather agreeable; they are of 

 Tertiary age. The oils of Jaj)an, differing much among them- 

 selves, have as a common character an odor quite different from 

 the Pennsylvania oils. So the j)etroleums of the Caspian, of 

 India, California, etc., occuring at different geological horizons, 

 exhibit a diversity of physical and chemical characters which 

 may be fairly supposed to depend ujoon the material from which 

 they have been distilled. The oils in the same region, however, 

 are found to exhibit a series of differences which ai-e phiinly the 



