MUSEUM OF COMPARATIVE ZOOLOGY. 



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Akerly did not decide between the igneous or aqueous origin of the 

 trap of the Palisades (62). 



For New Jersey, H. D. Rogers wrote in 1836 that the molten trap 

 had burst up through nearly parallel fissures, after the sandstone had 

 been tilted : the eruption caused no further change in the position of 

 the strata, but produced certain changes in their contents and structure 

 (a, IGO): and in 1840, "the protrusion of the trap, the formation and 

 deposition of the conglomerate, and the elevation and final drainage of 

 the whole red sandstone basin, have hardly been consecutive phenomena, 

 so nearly sinudtancous appear to have been these changes." ((?, 171.) 

 Cook considers the trap sheets intrusive, {b, 176, 200, 337; c, 32, 34.) 

 Husscll gives good evidence of the intrusive nature of the Palisade sheet 

 (d), and claims to have proved the same origin for the First Newark 

 Mountain (a). Wurtz regards the Palisade sheet as nactamorphosed in 

 place from sediments ; he is doubtful if this origin apply to the Newark 

 Mountain as well (101). 



Pennsylvania gives so few good opportunities for observation, that 

 little has been written about its trap sheets and dikes, though they ai-e 

 extensive and numerous. Many are omitted from the State Geological 

 Map, 1858. Farther south, dikes seem much commoner than sheets; 

 and so far as I. have learned, there have boon only two suggestions of 

 contemporaneous overflow for any of the trap southwest of New Jersey. 

 The lirst, by J. 13. Gibson, was written in 1820 and published in 1825 : 

 *' That the trap may have been deposited on the sandstone by a volcano 

 before the present continent was elevated above the level of the sea, 

 would bo a more plausible supposition ; but it would be altogether gra- 

 tuitous." (Observations on the Trap Rocks of the Connewago Hills, 

 159.) The second was by H. D. Rogers, who wrote : "In certain cases 

 the entire length of each middle secondary bolt seems not to have been 

 uplifted to the sea level before the commencement of the trappcan erup- 

 tions; and those tracts which remained thus submerged are seen to con- 

 tain, interstratilied, as it were, with the later sedimentary deposits, those 

 sandy volcanic tuffs or subaqueous sedimentary forms of trappcan matter 

 which constitute the link between the exclusively aqueous and igneous 

 masses.'* (g, II. 762.) The context shows that such cases were regarded 

 as very exceptional in Pennsylvania at least : no definite localities are 

 given. "Overflow," as used by H. D. and W. B. Rogers (g, 11. 670 ; 6, 82) 

 and Lesley (a, 133), seems to refer to eruptions after some erosion of 

 the Triassic strata, so that the trap should lie unconformably on the 

 sandstones : they all seem to regard the trap as post-Triassic, with slight 



exception. 



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