THE HOLYOKE SHEET, 449 



Seen from the west, a marked depression and eastward recession of a 

 central section of the trap ridge is manifest, and as the smaller eastern bed 

 was finely faulted at points opposite to the extremities of this section and 

 the part between the faults moved east, these faults Avere prolonged west- 

 erly to explain the structure of the main bed, and I was able by later 

 study to locate them quite exactly on the ground. The northern is very 

 plainly marked in the western boundary of the main sheet, which bends 

 sharply east in an acute angle. 



THE FAULTS AT MOUNT TOM AND SOUTHWARD. 



Viewed from the south, Mount Tom is a table mountain, having a nearly 

 horizontal sheet of trap, 300 feet thick, resting upon a great pede-stal of sand- 

 stone which rises about 900 feet above the sea, with vertical scarp on west, 

 south, and east. At the foot of the eastern scarp a fault runs very obliquely 

 to the coui-se of the bed, about N. 35° E., and on the west of this fault the 

 mass is raised about 650 feet, so that if one stands on the road south of the 

 mountain the trap seems to come to a sudden end in Mount Tom, but turn- 

 ing eastward one can traverse its whole width and can follow it thence 

 south continuously across the State, and can trace the sandstone north in 

 a sharp triangular projection sent in between the two sections of the trap 

 by the displacement of the fault. This eastward-facing bluff of Mount 

 Tom sinks northwai'dly; but where the fault crosses the river and makes 

 the westward-facing bluff of Mount Holyoke the throw is about the same. 



At Titans Piazza we have strike N. 85° W., dip 22° S., which would 

 carry the base of the trap far below the level of the river at Titans Pier. 

 Since, then, the lower contact appears at the water's edge at Titans Pier, 

 another fault must pass to the east of this point, running between the pier 

 and the piazza, with an uptlu-ow on the west of about 625 feet. 



The new Holyoke reservoir lies just across the north line of Holyoke 

 in Northampton, and the wood road from its north end soon crosses a brook 

 running north, and here the gray sandstone rests against the main sheet of 

 trap, which is brecciated for several feet down and cemented by a fine, 

 light-gray sand at the contact on the fault. 



The slickensided fault-wall has been well exposed by the cut on the 

 electric road just south of the lower station of the cable road onto Mount 

 Tom; and about 5 rods south along the fault, where a small brook comes 



MON XXIX 29 



