2^4 Longwell — Structure of the Triassic 



crops. Saltonstall Ridge is the surface expression of the 

 middle or main trap sheet. Immediately north of Lake 

 Saltonstall the ridge curves abruptly eastward, following 

 the strike of the rocks, and at the location of the tunnel 

 the strike is almost directly east, with the dip toward the 

 south. Throughout its length the ridge traces the rim 

 of an incomplete structural basin, aptly referred to by 

 Davis 2 as a "dish" or " half -boat-like basin," which 

 terminates abruptly against the great fault bounding the 

 Triassic area on the east. 



The tunnel cuts the ridge in the direction N11°E. 

 South of the brick conduit indicated in the section (fig. 2) 

 there is no lining except for a few feet in a shatter zone 

 between stations 10 and 11. Thus the solid rock is 

 exposed practically without interruption for a distance 

 of 1800 feet. In the cut near the south portal coarse 

 Triassic sandstone is exposed, dipping 35° south. The 

 top of the lava sheet is amygdaloidal, and extreme decom- 

 position prevents location of the exact contact with the 

 overlying sandstone. Inside the portal the amygdaloidal 

 rock gives way to massive basalt, and this is succeeded 

 by diabase as coarse-grained as that from large intrusive 

 sheets such as West Bock. Columnar jointing is evident, 

 but not prominent. Near the base of the sheet the dia- 

 base again grades into dense basalt, which rests on hard 

 conglomeratic sandstone. The surface of contact, 

 sharply defined and essentially a plane, is inclined 35° 

 to the south. Thus the tilt of the lava sheet may be taken 

 as 35°, and its computed thickness, allowing for the 

 obliquity of the tunnel, is 475 feet. Possibly this figure 

 is slightly in excess of the actual thickness, because of 

 small normal faults which are not obvious in the massive 

 igneous rock. No evidence was seen that more than one 

 now is represented, but such evidence might easily escape 

 notice in the narrow section given by the tunnel. 



No metamorphism is apparent below the contact, but 

 such effect is not expected, inasmuch as the topmost sedi- 

 mentary beds are of conglomerate and coarse sandstone. 

 The beds exposed between the lava sheet and the brick 



2 Davis, W. M.: Structure of the Triassic Formation of the Connecticut 

 Valley, U. S. Geol. Survey, 7th Ann. Kept., p. 478, 1886; also, The Triassic 

 Formation of Connecticut, U. S. Geol. Survey, 18th Ann. Eept., part 2, p. 85, 

 1897. '* ' 



