268 BULLETIN OF THE 



former is certainly as much as two hundred feet at some points ; and 

 their general trend is east. A rough but distinct columnar structure 

 is seen at right angles to the sides. Their junction with the sandstone 

 is somewhat irregular; but this is natural, as the latter has no well- 

 mai'ked joint planes. 



East Rock shows two patches of lower sandstone on its southwestern 

 face ; they are very similar to that described under West Rock, except 

 that about eight feet below the conformable junction, a spotted appear- 

 ance has been produced in a layer of purplish granitic sandstone. The 

 trap is also like that of West Rock. Ascending by the road on the 

 southern side of the hill, and when the eastern slope is reached turning 

 across a field into the wood,- a very strongly baked granitic sandstone 

 is found within a few feet of the fine compact trap : the sandstone is 

 very dense and much more crj-stalline than any found elsewhere in 

 the Connecticut valley, but it rapidly loses this character, and becomes 

 soft and fragile farther down the hill, like that found on the back of 

 West Rock. 



A number of dikes (fig. 44) may be found by crossing the river from 

 New Haven by Tomlinson Bridge, and continuing half a mile along 

 Forbes Avenue toward East Haven. These were long ago described by 

 Hitchcock {h, 56), but his figure (here copied, fig. 2) makes them much 

 too regular; they vary in strike and dip as well as in thickness, and 

 their sides are uneven. They are not at all amygdaloidal ; their bak- 

 ing extends one or two feet into the adjoining granitoid sandstone. 

 The second cut on the Shore Line Railroad east of Fair Haven shows a 

 similar dike; but the third cut is in a coarse granitic sandstone that 

 makes a strong ridge by itself, being hard enough to stand up between 

 the softer shaly sandstones on either side, though not visibly aided by 

 any igneous rock. The ridge west of Saltonstall's Lake (Percival's 

 E, L) is cut at a low gap by the same railroad, and shaly sandstones 

 are exposed, conformably covered by trap, and baked to white quartzite 

 immediately at the junction, as at West Springfield. The line of con- 

 tact is broken at one point by a very small fault, displacing the shales 

 and trap equally, and evidently of later date than the eruption. The 

 trap is brecciated at certain points, and is generally uneven in its joint- 

 ing, and its upper surface along the lake shore is very amygdaloidal. 

 No overlying sandstone could be found. 



There is fairly good evidence that the natural gap, cut deeper by the 

 railroad, results from a transverse fault of ten or twenty feet, with the 

 throw eastward on the southern side. 



