EVIDENCES OF FAULTING. 523 



anterior trap sheet, the anterior shales, the main trap sheet, the posterior 

 sandstones and shales, the posterior trap sheet and a greater or less thick- 

 ness of upper sandstones and conglomerates. It is perhaps possible to 

 invent a S3^stem of very special conditions by which the monoclinal 

 strata can be supposed to end underground against a great cliff or steep 

 land-slope, at whose base lay the water body in which the}^ were accumu- 

 lated ; but the essential conditions of this invention are so peculiar, so 

 complicated, and so little in accord with the other features of the region 

 that they are soon discarded ; hence the dip lines by reasonable infer- 

 ence, as well as the strike lines by observation, must be interpreted as 

 terminating against a fault. On the other hand, the northern part of 

 the border has few neighboring Triassic outcrops. The evidence of fault- 

 ing is less distinct there than further south ; yet on making a general 

 section on true scale across the Triassic lowland it is difficult to account 

 for the disappearance of the strata unless cut off by a marginal fault. 



Fault-line Valley. — A narrow, meadow-like depression, followed more 

 or less by streams, but here or there rising in low divides, is traceable 

 more or less continuously along the boundary line between the adjoining 

 outcrops of Triassic and crystalline rocks. There is no reason for this 

 valley except the occurrence of a belt of shattered rock, such as is com- 

 monly produced on strong fault-lines. Sometimes the depression or 

 fault-line valley is a quarter of a mile in width, being then, as a rule, 

 adjoined by the weaker members of the Triassic series. Sometimes it is 

 nierel}^ a narrow pass between close a[)proaching slopes on either side ; 

 this being the case where the harder Triassic members confront the 

 bolder crystalline slopes. The change of the depression from one form 

 to the other is comparatively systematic, as may be seen near the north- 

 east end of the Totoket crescent. The crystalline slope here maintains a 

 fairly regular course on the eastern side, but the western side of the depres- 

 sion varies with the resistance of the Triassic member ending upon it. 



North of the Connecticut river the marginal depression, like the other 

 features by which the fault-lines are located, is much less distinct than to 

 the south. The absence of the marginal valley is presumably an expres- 

 sion of the two prevailing features thereabouts ; relatively weak Triassic 

 rocks, allowing an effective baselevelling of the whole area and a plenti- 

 ful distribution of drift, almost ol)scuring the rock topogra2)hy. 



Associated Dislocations. — The disturbances which we associate with the 

 X)roduction of the greater marginal faults are found in both the Triassic 

 and the crystalline areas. The former will be mentioned first. 



In the Pond Rock crescent the ridge formed by the main trap sheet is 

 repeatedly offset by small faults as it approaches the crystallines at its 

 southern end, each offset being marked by a gap or notch in the ridge. 



LXXIII-BuM- GKor. Soc. Am., Vol. 5. 189.3. 



