CONNECTICUT RIVER SANDSTONE. 353 



feet above sea level, instead of 4,000 feet below, as in tlie remaindei* of the 

 basin. This I call the Amherst area. Just west of this area, from Turners 

 Falls to Northampton, and south of it across Hampden County, this sub- 

 stratum has not been reached by borings from 000 to 3,700 feet. On this 

 elevated substratum rest the g-reat conglomerate masses of jVIount Toby 

 and Gill, rising several hundred feet above the adjacent plateau area on 

 the east, from which they must have received their material. 



It seems to me probable that this block, bounded by the Leverett fault 

 on the east, the Mount Tom-Northfi.eld fault on the west, and the Mount 

 Holyoke faults on the south, has experienced a later movement of elevation 

 in opposition to the prevailing sinking of the valley blocks, and that this 

 explains its present elevation and the present height of Mount Toby. 



This eastern border fault follows the line taken by the railroad east of 

 Mount Tobv; and on the east of the raih-oad the conglomerates occur at 

 the railroad level, while on the west of the railroad the junction of the 

 conglomerate on the old quartzites below is about !')0 feet above tlie rail- 

 road ; so the upthrow of the conglomerate along its eastern edge by this 

 one fault must be 50 and may be nearly 100 feet. 



The uijthrow of the Mount Toby block on its western edge is also 

 made probable by the following considerations: 



The great Northfield fault, continuing south from the mouth of i\lillers 

 River, seems to pass beneath the Connecticut between Mount Toby and 

 Sugar Loaf. The flat top of Sugar Loaf in this exposed place seems to be 

 due to the fact that the Deerfeld trap sheet formerly capped it and has 

 been removed by erosion (probably near the end of the Glacial period) so 

 recently that the mesa form remains. The dip of the trap sheet in Mount 

 Toby is 15° E., and its distance from Sugar Loaf is 5,610 feet. With tliis 

 di}) it would be carried over Sugar Loaf, 936 feet above its summit. But 

 the dip m Sugar Loaf is 8° E., and, allowing the dip to change at the 

 fault, that is, about midway between the present trap outcrop and the mesa 

 top, the height of the trap sheet over Sugar Loaf, if there were no fault- 

 ing, would be 575 feet, and this latter number would be near the true 

 amount of the upthrow on the east side of the fault. 



MON XXIX 23 



