540 THE POPULAR SCIENCE MONTHLY, 



pregnating limestones. The last combination produces the mineral 

 commonly called asphalt. When the bitumen contained in any of 

 these substances is chemically isolated, it appears always a nearly 

 identical substance, in composition, consistency, and appearance, ex- 

 cept that the empyreumatic odor that characterizes it may become alli- 

 aceous in volcanic countries. Asphalt is doubtless one of the most con- 

 siderable and valuable of the forms in which bitumen appears. It is a 

 soft limestone, naturally and closely impregnated with that substance. 

 When a specimen of it is examined under the microscope, each grain 

 of it appears to be immersed in a pellicle of pure bitumen, by which 

 it is cemented to the adjoining particles. It is thus a species of very 

 fine-grained bituminous conglomerate. When a lump of this rock is 

 heated to a temperature rising from 176 to 212, the pellicle of bitu- 

 men is melted, the cohesion of the asphalt is destroyed, and it crum- 

 bles into dust. If it is taken while it is still hot, or if it is heated again 

 after it has become cool, and strongly compressed, the particles will 

 adhere again, and the stone will recover, after cooling, precisely the 

 consistency and appearance it had originally. The employment of 

 compressed asphalt for i^avements is founded on this property. 



Asphalt, or bituminous limestone, is generally found in the Jurassic 

 strata, in regular beds of a lenticular shape, which are uniformly cut 

 in two by a stream of w^ater. Sometimes the bed is single, at other 

 times it is multiple ; there are formations containing seven beds, 

 one above the other, and distinctly separated by strata of white lime- 

 stone. 



Diiierent views prevail respecting the origin of asphalt and the cir- 

 cun^stances under which it is formed. Some believe that the bitumen 

 was already in existence when the calcareous formation took place, 

 and that the particles of limestone were deposited in a bituminous sea. 

 Others consider that the bituminous matter is derived from the organic 

 matter associated with the shells that have furnished the carbonate of 

 lime ; and other more hazardous hypotheses have been advanced. A 

 careful observation of asphaltic formations has led me to adopt what 

 appears to me to be a more plausible theory. 



It is permitted to suppose, from indications furnished by the study 

 of bituminous districts, that in some geological epochs, which have yet 

 been only imperfectjy determined, accumulations of organic matter, 

 buried under enormous masses of Jurassic limestone, and heated by 

 the central fire, became vaporized, and in that condition sought a pas- 

 sage through the crust of the earth (Fig. 1). In time the crust cracked, 

 and a fissure was formed. The bituminous vapors, compressed by in- 

 calculable pressure, forced themselves through the way that Avas opened 

 to them, and passed by such strata as were too compact to be pene- 

 trated ; but, when they reached the oolite, they found on either side 

 of the fissure beds of a limestone soft enough to admit of their impreg- 

 nating it (Fig. 2). As long as the pressure lasted, the bitumen con- 



