﻿344 W. 31. Davis — Structure of the 



in its several other areas, where the monoclinal structure is also 

 developed, early gave rise to the theory that the intrusion of 

 these rocks must have caused the tilting of the adjoining beds. 

 Whatever possibilities of disturbance such intrusions may pos- 

 sess, their action is certainly excluded from the greater part of 

 the Connecticut Valley area, for here nearly all the igneous 

 rocks are found to be contemporaneous overflows : the intru- 

 sive sheets form much the smaller share of the trap-ridges and 

 seem to be limited to the western border of the formation. 

 The overflows having taken their present position among the 

 strata while the formation still lay horizontal, could have had 

 no active share in disturbing it, but must instead have passively 

 suffered deformation with it. It is very probable that the in- 

 trusive sheets also took their present position in the sediment- 

 ary series before the formation was tilted and faulted. Similar 

 intrusive sheets are known among strata that still lie horizontal, 

 and that were no more disturbed at the time of intrusion than 

 was necessary to give place to the lavas that were driven be- 

 tween them. No peculiarity of structure is found in the 

 neighborhood of these Triassic intrusions that is not found else- 

 where in the formation ; but on the contrary the structure that 

 they and their adjoining sandstones present is a remarkably 

 faithful imitation of the structure that is at once so peculiar 

 and so characteristic in the overflow sheets in other parts of 

 the formation ; and even if the disturbance in the neighbor- 

 hood of the intrusions be attributed to their action, this singu- 

 lar imitation remains to be accounted for. Indeed there is not 

 only an imitation but a correlation in the structure of the dis- 

 tricts occupied by the intrusions and overflows : the ridges 

 formed on their outcropping edges are all convex to the west; 

 where they are disjointed, their ends overlap or offset in a sim- 

 ilar fashion ; the overlaps or offsets of the West Eock range 

 being in what Percival called the advancing order (south end 

 of northern member overlapping north end of southern mem- 

 ber on the west), like the Hanging Hills and other southern 

 overflow ridges ; while the Barn-door Hills in Granby, which 

 seem to be the northern representative of the West Eock range, 

 overlap in receding order, like the northern half of the great 

 range that extends from the Hanging Hills to Mounts Tom and 

 Holyoke. In view of this, it can hardly be doubted that the 

 intrusive sheets as well as the overflows and bedded rocks once 

 lay horizontal, and that the whole mass of strata, igneous as 

 well as aqueous, was subsequently and passively deformed by 

 an external disturbing force. 



The idea that the whole monoclinal represents an unfaulted 

 mass, tilted to an almost constant eastward dip,* has not gen- 



*This is most definitely presented in LeConte's Elements of Geology, 1878, 

 441. 



