of the Eastern States. 245 



evidently occur in the bottom of the smaller side-arches, which 

 are bordered on one side by the rigid metamorphic rocks sur- 

 rounding the basin, on the other side by the upheaved portion in 

 the center. Such an upheaval, it appears to us, must result in the 

 fracture of the rocks along the sides of the estuary in lines nearly 

 parallel with the axis of upheaval. We know that elevations of 

 the earth's crust are often due to the expansion of the igneous 

 matter that exists beneath large portions of its surface. When 

 a fracturing of the rocks occurs above these reservoirs of lava, 

 the force that caused the upheaval is expended in driving the 

 molten rock upwards, into or through the super-imposed strata. 



This is what we conceive took place when the axis of crys- 

 talline rock was upheaved, which separates the Triassic areas in 

 New Jersey and the Connecticut Yalley. The trap, escaping 

 through the fractures in the metamorphic rocks beneath, was 

 forced upwards among the sandstone strata, sometimes opening 

 their layers and forming sheets of injected matter, at other times 

 traversing fissures that cut through the bedded rocks, and so 

 forming trap dikes. 



The protrusion of the trap could not have taken place until 

 the upheaval had culminated. As we cannot but believe that 

 the upheaval was gradual, and that the work of erosion must 

 to some extent have kept pace with it, the relative elevation of 

 the country would not have been much greater than it is at the 

 present time. For this reason, the trap sheets could never 

 have extended far beyond their present limits. Some of them, 

 as we have seen, never reached the level of the present surface 

 of the country. That these trap ridges have suffered consider- 

 able denudation, is shown by the vast amount of trap boulders 

 that are strewn to the east and south of the exposures of this 

 rock in the drift area, and by the fine light-colored earth and 

 angular blocks of trap, that are found along their flanks in the 

 driftless region to the southwest. 



That the force which opened the layers of the sedimentary 

 rocks and forced out the trap in a molten state, was sufficient, 

 when confined beneath the surface, before the fractures in the 

 in the metamorphic rocks occurred, to upheave the central por- 



