348 W. M. Davis— TriassiG Trap Bocks. 



rim erescentic form, first fully recognized by Percivnl. 

 with their bold convex side toward the up-slope of 'the mono- 

 clinal, and their horns toward the down-slope. The existence 

 of such folds is well proven for several cases by finding that 

 the strike of the nemhboiiue. -ami-tone i^ do^eh paialle] to 

 the trend of the curve' I ridge, and that the dip is directed in 

 toward the center of the curve. This has been shown but not 

 fully appreciated by several observers. 



Faults are known by the reappearance of certain strata, or 

 series of strata. In this way it may be made very probable 

 that .the Hanging Hills sheet reappears in Lamentation Mount- 

 ain, and again in the several strong ridges as far south as Lake 

 Saltonstall; the face of most if not all of these ridges being 

 characterized by sandstone, amygdaloidal trap, limestone, shale 

 and heavy trap, always occurring in this (ascending) order: 

 smaller examples could also be named. But no sufficient 

 reason has been found to show why a general monoclinal tilt- 

 ing should have taken place. 



The New Jersey and Massachusetts areas have few folds and 

 faults shown by trap ridges ; they are made on a larger pattern ; 

 but faults mav occur, unperceived, in their sandstone. Con- 

 necticut is decidedly the best field for the study of all the 

 points above mentioned in the history of the Triassic forma- 



The age 



of the intruded sheets has been referred to as inde- 





it is not as yet susceptible of good limitation. The 





srs, who considered all the trap intrusive, olten 



looked on 



it as the agent of disturbance of the sandstone: 





rs, as Dana and Russell, regard it as intrusive after 



the tilting 



of the sandstones had been accomplished by some 



other force. 



As opposed to these views, I would present the 



following c 



•vidence of its intrusion before the tilting and per- 





■ contempora of the over- 





e dike, mid the intruded sheets, where seen to break 



across the 



strata, show irregular or ragged edges, as above 



noted : the: 



ir intrusion therefore took place' before the produc- 





ts, now verv distinct in manv parts of the sandstone. 





probably before the tilting, for the joint-making 









during or ; 



ifter the tilting, thee ',s no reason why they might 



the base of the formation, close to the adjoining cry 

 rock: and the only reason that f can assign for so pe 

 limitation is that the interbedded intrusions took pla 

 under a considerable pressure of overlying rocks, as < 



