168 J. D. Dana — Additional Observations on the 



trap, parallel with that of the Mt. Tom Ridge ; and that the 

 two were raised together from their original horizontality 

 along with the sandstone formation in which they were inter- 

 calated, into a monoclinal of 20° to 25°. The apparent 

 satisfactoriness of this explanation is the strongest point in 

 Mr. Davis's hypothesis. 



The parallel courses of the two unlike belts has always 

 seemed to me difficult to understand. But supposing the 

 Mt. Tom ridge to be of laccolithic origin, like West Rock, the 

 view I still believe to be most probable for reasons I have 

 stated, the new facts from West Rock suggest an explanation : 

 that this subordinate amygdaloidal belt of trap was produced 

 by a lateral discharge from the dike of the laccolithic 

 Mt. Tom belt, when the laccolithic discharge was nearing com- 

 pletion. This would account for the close relation of the two 

 in position. Moreover the trap of such a dike would be sure 

 to be amygdaloidal ; for the north and south line of dikes of 

 the Mt. Tom Ridge, situated along the western side of the 

 Connecticut valley trough, would have been, from the com- 

 mencement of the outflow, a barrier to the eastward or south- 

 eastward flow of subterranean waters descending from the 

 north and west, so that the accumulated subterranean stream 

 would have been large. To the eastward, the subterranean 

 waters of the valley would have been divided up by the 

 parallel trap dikes, for the hydration and vesiculation of other 

 such branch dikes. 



The value of the two hypotheses as to the origin of the 

 trap belts — the dike or intrusive and the monocline — can be 

 tested in two ways. One I have already referred to : the 

 removal of the trap debris making the long east-and-west talus 

 of the Mt. Tom ridge, near Meriden, exposing the sandstone, 

 as done for the east-and-west talus along the similar south 

 front of West Rock near New Haven. The sandstone may 

 be, as it is at New Haven, upturned beneath the trap-mass, 

 without conformability between the two rocks. If this were 

 found to be the case, there would, be no further ground for 

 doubt as to the upturning before the outflow. 



The other method is by boring. A boring carried down 

 through the sandstone at points a short distance to the west of 

 the Mt. Tom ridge would pass through, if the monocline 

 hypothesis is the true one, at no great depth — probably within 

 1000 to 2000 feet of the surface — a layer of compact trap 200 

 to 300 feet thick, and then, after a little more sandstone, a 

 layer of amygdaloidal trap ; and not so, if the dike theory is 

 correct. This test, repeated in other parts of the Connecticut 

 valley in Connecticut and Massachusetts, would give conclu- 

 sive facts. 



