376 E. 0. Hovey—Trap Ridges of the 



an abstract of the views of most of the writers who have dealt 

 with the Triasssic trap ; those, therefore, who desire to inves- 

 tigate in detail the history of the discussion are referred to his 

 article. In 1833, Pres. E. Hitchcock supposed that the trap 

 ridges were overflow sheets contemporaneous with the deposi- 

 tion of the sandstone ;* but afterwards he modified his views 

 so far as to hold that some of the sheets were intrusions.f 

 H. D. Rogers claimed that the trap of the New Jersey Trias- 

 sic region was erupted " through a series of nearly parallel fis- 

 sures in the strata, and after their consolidation and subsequent 

 disturbance."^; Prof. J. D. Dana adopts this theory to account 

 for the trap of the Connecticut valley Triassic.§ Most writers 

 on the subject agree to the latter view, but Prof. Davis sus- 

 tains the former, taking the ground that, while some of the 

 ridges are intrusions, most of them are overflows, and that all 

 were ejected before the tilting of the strata took place ;|| and 

 further, that the present tilted position of the trap is due to 

 monoclinal faulting of a sandstone-and-trap formation. 



In the New Haven region the West Rock range, Pine and 

 Mill Rocks, and the East Rock series of ridges are regarded by 

 Prof. Davis as intrusive sheets and dikes.^f The trap ridges in 

 the eastern part of the town of New Haven, already described 

 as the southern members of the third western range, would be 

 recognized as dikes by any observer. In Connecticut it is only 

 with regard to the great ridges which lie in the middle and 

 eastern part of the Triassic area, and extend northward into 

 Massachusetts that there is doubt as to the intrusive or extru- 

 sive origin of the trap. Pond Rock is one of these ridges ; and 

 this ridge, together, with its associated ranges, has been taken 

 by Prof. Davis to be typical of the whole Connecticut system.** 

 He says that Pond Rock is an overflow sheet on account of 

 "its small metamorphic effect at the base, its decided amyg- 

 daloidal texture on its back or upper surface, its irregular and 

 brecciated structure, and its alteration and hydration. "ff The 

 following facts bear on these arguments. 



At the section exposed in the railroad cut the underlying 

 sandstone was indurated for four or five feet from the trap. 

 The thickness of the trap sheet here is about 8-i feet. The ex- 

 tent of the induration is not proportionally much different from 

 what is shown at undoubted intrusions, but I have already 



* Rep. on Geol., Min., etc., of Mass., 1833, p. 243. 

 f Final Rep. on Geol. etc, of Mass., 1841, p. 653, but cf. p. 526. 

 ± Rep. on the Geology of New Jersey, 1836, p. 160. 

 § Manual of Geology, 1880, p. 423. 

 | This Journal, III, xxxii, 344, 1886. 

 «[[ Seventh Ann. Rep't U. S. G. S., p. 463. 



** Ibid. p. 346. See also 7th Ann. Rep. U. S. G. S., p. 478, where special stress 

 is laid upon Pond Rock. 



ff Bull. Mus Comp. Zool., vii, 269. Cf. this Journal, III, xxxii, 347, 1886. 



