302 



DYNAMICAL GEOLOGY. 



and the latter between Cretaceous ; and over the Cretaceous lie Tertiary 

 beds. 



It follows, from the conditions represented, that the ascensive thrust of 

 the lava was so powerful, that in spite of friction along the passage and the 

 density of the lava, it flowed upward for an unknown number of miles to the 

 laccolith level ; and then had energy enough left to lift, in the case of the 

 laccolith lowest in geological level, a mass of beds 10,000 feet or more thick 

 and 2-25 in average specific gravity (equivalent in pressure to 675 atmos- 

 pheres) to a height of 5000 feet. Some accession to the force, however, may 

 have come from vapors derived from subterranean moisture, or from waters 

 encountered on the way up. As Mr. Gilbert states, the intrusion of the lava 



274. 



276. 



SlS«!. 



275. 



277. 



274, Ideal outflow of the main East Rock dike; 275, actual outline of trap in the same, with an eaBtern 

 supplementary dike; 276, same in a second East Rock summit, called Indian Head ; 277, upturned sandstone 

 (with talus covering part of it) underneath the trap of West Rock along a transverse section. D. '91. 



laterally into a chamber widened the area of pressure, and thus enabled it, 

 on the principle of the hydraulic press, to accomplish the lift by very slow 

 steps of progress. 



