Jura-Trias trap of the New Haven Region. 165 



Art. XXIII. — Additional observations on the Jura-Trias 

 trap of the New Haven Region • by James D. Dana. 



Iisr connection with my description of the south front of 

 "West "Rock, the annexed figure of the eastern part of the ex- 

 posed sandstone with a portion of the base of the overlying 

 sheet of trap is introduced.* It is repeated here in order to 

 mark from it the localities of two new observations of interest, 



I. The exposure of sandstone represented in the above 

 figure owes about eight feet of its height to the removal of 

 overlying trap by the quarry men. The trap quarry (see Plate 

 VII of my former paper) extends from the point E eastward 

 for nearly 140 yards. At a spot about 100 yards east of this 

 point and 16 feet below it in level, the removal of the debris 

 from the floor of the quarry has exposed the top of another 

 ledge of upturned sandstone. The escape of the trap from 

 the dike, therefore, was not along the sloping surface of the 

 upper layer exposed at E, but along another about one hun- 

 dred yards farther east. 



II. About 90 feet west of the point E in the preceding 

 figure, and 40 to 50 feet above the quarry-road, the talus with 

 the partly concealed sandstone is crossed by a five-inch dike of 

 trap. It was first observed last autumn by Professor W. O. 

 Crosby. It looks from the quarry-road like a slight break in 

 the steep surface. (On the phototype, Plate YII, referred to 

 above, it is very faintly indicated across the bed of sandstone, 

 just one inch from the sandstone point E.) 



The little dike is a branch from the underside of the main 

 sheet of trap. It makes a sinuous line of outcrops extending 

 westward along the talus for about 40 feet, and then disappears 

 under the debris. In this distance the dip of the outcrop is 

 about 18 feet ; but the true dip of the sheet is northwestward 

 about 20° to 25°, as nearly as could be ascertained. Toward 

 its junction with the main mass of the West Rock trap, it is 

 reduced, for nearly a yard of its length, to two strands hardly 



*This Journal, xlii, 102, 1891. 



