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MUSEUM OF COMPARATIVE ZOOLOGY. 



301 



H. D. Eogers 



last are the most extended effects of the trap that I found. In the West 

 Springfield raih-oad cut (C), the baked saiidatonc for a quarter or half 

 inch from the contact was almost white on fresh surfaces. 



The absence of red color in strata near a contact has been noted by 

 several observers. Silliman described the sandstone close under the trap 

 of Eocky Hill by Hartford as gray or white (e, 125). Pcrcival found 

 that the sandstones near the trap were "sometimes discoloured greenish 

 or brawn, and at other times their usual reddish color is apparently dis- 

 charged, leaving them nearly white" (319, also 436): in a few cases 

 they are black or red (437). Lycll found some of the shales turned 

 white below the traps of the Richmond coal' field (c, 271). 

 described the shales near the traps as dull brown or pm^ple {g^ 673, 

 678). The change to black color as a consequence of baking is men- 

 tioned by Cook (6, 20G, 212) and Kerr (6, 147). In view of these facts, 

 it is impossible to consider the prevailing color of the New Ked Sand- 

 stone in any way dependent on the action of the trap after the deposit 

 of the sandstones. How much effect the contemporaneous eruptions may 

 have had upon weathering and color, is an open question. 



In regard to the effect of the trap on the hardness of the sandstone 

 the most excessive views were those of H. D. Rogers, who considered 

 most of the good building sandstone in New Jersey hardened by baking 

 (c, 157) ; and of Whelpley, who thought that the sandstones had covered 

 a broad area in Connecticut, but had been preserved only near the trap, 

 where it was hardened (62). But here, as before, it may be urged that 

 as hard sandstones occur plentifully in regions free from eruptive rocks, 



it is more probable that variations in the hardness of the sandy and 

 shaly Triassic strata, except those close to the trap, are due to the un- 

 equal action of the ordinary processes of consolidation. Whatever dis- 

 tinct hardening effect was produced by the trap, it generally extended 



but a short distance into the adjoining rocks, 

 than one hundred foct. 



probably in no case more 



Slight mineral alteration reached farther than 

 any other notable change. 



The marked difference between the effects of the intruded and the 

 overflow sheets has already been pointed out. The Palisade trap (H, J) 

 liardcned the adjoining sandstones and shales very distinctly for twenty 

 feet and probably farther, and in some of the more easily altered layers 

 produced a marked crystalline structure : this was equally apparent at 

 upper and lower contacts. On the other hand, the lower contact of the 

 ovei'ffows shows a very slight change from the normal sandstone. " Em- 

 erson describes the alteration under the Greenfield trap as extending 



