ORIGIN AND USES OF ASPHALT. 331 



drift, having been modified during subsequent geological changes, breaking up 

 and entirely changing their original depositions and distribution." 



In gathering material for this paper, I wish to make grateful acknowledg- 

 ments to Gen. George H. Nettleton for railroad courtesies, and to Messrs. Scott, 

 Grimes and Garretson, of Burlington, for special favors fitted to gladden the 

 heart of a scientist. 



ORIGIN AND USES OF ASPHALT. 



BY LEON MALO, C. E. 



Bitumen appears in nature as an accidental mineralogical product, under the 

 the most diverse and often most inexplicable conditions. It is found sometimes 

 in the native state, sometimes mixed with clays, sometimes as the cement of con- 

 glomerates, sometimes impregnating 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 sub- 

 stance, in composition, consistency, and appearance, except that the empyr- 

 eumatic odor that characterizes it may become alliaceous in volcanic countries. 

 Asphalt is doubtless one of the most considerable and valuable of, the forms in 

 which bitumen appears. It is a soft limestone, naturally and closely impregnatfd 

 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 

 i' 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 bitumen melted, the cohesion of the as- 

 phalt is destroyed, and it ciumbles 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 pavements 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 into by a stream of 

 water. Sometimes the bed is single, at other times it is multiple ; there are forma- 

 tions containing seven beds. One above the other, and distinctly separated by 

 strata of white limestone. 



Different views prevail respecting the origin of asphalt and the circumstances 

 under which it is formed. Some believe that the bitumen was already in exist- 

 ence when the calcareous formation took place, and that the particles o'f 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. 



