AGE OF EJEPTILES. 



243 





throughout. The explanation is this: Five successive 

 layers of mud had been laid down of this thickness, each 

 one having partly dried before the succeeding one was 

 laid. The animal treading upon the upper one made an 

 impression through the whole five layers, which after- 

 ward solidified into rock, and so were preserved through 

 long ages to the present time. Dr. Hitchcock ingenious- 

 ly mounted the leaves in such a way that the book can 

 be readily opened or shut. 



348. Trap Rocks. Masses of these rocks have come 

 up among the Triassic rocks in the eastern part of this 

 country, many of them being lofty enough to be called 

 mountains. As examples of these, there are, in Massa- 

 chusetts, Mounts Tom and Holyoke ; in Connecticut, 

 East and West Rock at New Haven, and the Hanging 

 Hills of Meriden ; in New Jersey, Bergen Hill, and vari- 

 ous other elevations. I have already told you, in 230, 

 how these masses are formed. The effect which the 

 fused rock, as it was thrust upward, produced upon the 

 sandstone is plainly to be seen in many localities. The 

 difference between the sandstone close by the trap and 

 that which is away from it is as distinct as that between 

 a brick so burned as to be very hard and one which has 

 the ordinary hardness. The ridges and dikes of trap are 

 arranged with some system, extending in certain general 

 directions as mountains do. This may be seen in Perci- 





