34:6 W. J/". Ihiv'tH- Trhts-vc 7 nip Hocks of 



None of the dikes are surely known in Massachusetts, but 

 Connecticut gives numerous examples at Wallingford, East 

 and New Haven and elsewhere; in New Jersey they are rarely 

 seen, but farther southwest they are frequently mentioned. 

 None that I have seen are amygdaloidal ; their sides are gen- 

 erally uneven and are sometimes very ragged, implying an 

 eruption before the formation of joints in the sandstone; their 

 metamorphic effect on the adj . at, so far as 



determined; not extending more than half a foot to two feet 

 from a ten-foot dike, or eight to twelve feet from a hundred- 

 foot dike. The largest dikes that I have seen are Mill and 

 Pine Rocks, New Haven : they are one to two hundred feet 

 thick, of medium coarse texture in the middle, fine at the 

 sides, compact throughout ; they break through gently in- 

 clined, coarse sandstone strata about at right angles to the bed- 

 ding. and have a rough transverse columnar structure. It is 

 probable that few of the supply-dikes which fed the largest 

 sheets are yet laid bare. 



The intruded sheets are recognized by a distinct meta- 

 morphic effect on the strata above as well as below them, and 

 he absence of amygdaloids; they very probably cut across 



— :„*„ -U.,,. ivt- * a i wa ys be 



» be evenly 

 ley are li 

 the dikes in being younger than the strata which enclose them, 

 but their age is not surely determinate, as will be shown be- 

 low. The range from West Rock northward from New Haven, 

 and the Palisades along the Hudson are the largest and finest 

 nples : the latter shows as m tic effect on 



i those below it ; the former has pro- 

 there is no doubt of its intrusive 

 character. Other smaller intrusions are found along the Dela- 

 ware. It may be here noted that the Triassic trap sheets have 

 been regarded as intrusive by nearly all authors who have 

 written apon them; Mr. I. C.' Russell has recently stated this 

 view of the origin of the First Newark Mountain, N. J. (in- 

 correctly, as 1 think), as well as for the Palisades. Professor 

 Dana considers all the sheets intrusive, and does not mention 

 overflows in his Manual of Geology, 1380. 



The overflow or contemporaneous origin for the n 

 was first clearly stated by E. Hitchcock, 1833 and 1841 ; it 

 was later advocated by Principal Dawson for Nova Scotia, and 

 recently by Professor Emerson of Amherst,* but it has never 

 bad many advocates. 



The overflow sheets are known from being commonly very 



amygdaloidal on their back or upper surface, and sometimes 



* See this Journal for September, 1882. 



