MUSEUM OF COMPARATIVE ZOOLOGY. 307 



of tlie sandstone ; for as a trap sheet made its way obliquely upward be- 

 tween tlie layers, the cover of the sandstone must have been cracked at 

 the ends, allowing the trap to rise there and so turn the extremities of 

 its outcrop with the dip of the bedded rocks (e). This was agreed to by 

 Silliman, Jr. Whelpley thought the crescent ridges determined by the 

 form of the fissures in the old rocks below the sandstones (*j4). Dana 

 considered the curved ridges as marking curved fissures, characteristic 

 of certain eruptions, and further added that the agreement in form of 

 these ridges with more prominent features of the earth confirms " the 

 view that ranges of mountains and islands correspond to ranges of 

 fissures" (a, 391, 392) ; but later (6, c, 21) he essentially follows Rogers's 

 explanation, and further says that the tilting was caused by the subsi- 

 dence of the estuaries, and is "without evidence of folds" (c, 421). 

 Wurtz considers that subsidence went on to a small amount during 

 deposition, and was fastest along the axis of the belt. "Such slight 

 inward inclination of the beds on both sides of the basin, explains the 

 crescent form of the edges of the sheets of trap. The flow or propaga- 

 tion of the metamorphic agent being thus governed." (102.) Russell 

 calls the curves " lines of least resistance," which were naturally and 

 necessarily chosen by the eruptive trap (c, 241). Cook says, "The prin- 

 cipal changes of dip appear to be, in some unexplained way, connected 

 with the direction of the trap ridges, and are near them " (c, 29). All 

 of these explanations, based on the intrusive origin of the trap, fail when 

 the sheets are found to be overflows. 



The value of the ovei-flow sheets as marking horizons in the sandstone 

 formation is thus very considerable, and will in time lead to closer meas- 

 ures of thickness than have yet been possible. The peculiar restriction 

 of the localities of footprints in the Connecticut valley to the strata 

 along the back of overflows is well shown by Hitchcock in the map in 

 his Ichnology : it is evidently connected, as he suggests, with the appear- 

 ance of volcanic islands and shallow waters in the old estuary. 



6. SUMMARY. 



The Triassic strata were deposited nearly horizontal in narrow estu- 

 aries not greatly exceeding their present area : some degi-ees of their dip 

 may be the result of original oblique deposition, but this is insufficient 

 to explain all of it. During their accumulation, extensive and repeated 

 eruptions, very possibly from fissures, poured sheets of trap over their 

 surface, to be buried under later deposits ; and at the same time, or later, 



