THK POSTBlilOlt SHEET. 475 



calcite, in wliicli the nmiul and often confluent ssteaui holes are bounch'd hy 

 very thin walls, which appear as black lines surroundino- the calcite fillings 

 and contain only twinned plagioclase rods. The sandstone also contains 

 crackled fragments of bottle-green glass in which rndlike crystals of plagio- 

 clase of two sizes appear. The fragments are thus like the white trap found 

 a mile farther north, on the brook north of the station of tlui Mount 'I'om 

 Railroad (see p. 3G5). 



THE I'OSTEllIOK DIKE ACROSS HAMPTON COUNTY. 



One can trace the trap for a short distance farther south, hardly t<> the 

 town line, as everything is heavily covered with alluvium. The bed then 

 disappears and seems to be concealed for nearly 3 miles by the throw of 

 the Holyoke fault. It reappears again on the road from Holyoke to 

 Wright's pond, with a thickness of 33 feet, and can thence be traced south- 

 ward to the excellent sectioii made by the Holyoke and Westfield Railroad, 

 where it is 120 feet thick and divided into two beds. The lower of these 

 (resting on the fine red sandstone, dipping 15° W., and not perceptibly 

 altering it) is 53 feet thick, very coarse-columnar in its main mass, with 8 

 to 10 feet amygdaloidal above, while the upper bed rests directly uj)on the 

 lower one and is amygdaloidal at its surface for 15 feet in thickness, and 

 the whole is covered by a fine-grained red sandstone. 



This thinning out toward the north may indicate that the bed is not 

 continuous across the covered area; on the other hand, the double character 

 of the bed is repeated as in the bed to the north. South from the railroad 

 crossing the bed appears as a continuous ridge, increasing in altitude until 

 it reaches the Boston and Albany Railroad, where another fine section is 

 exposed east of the Tatham station, which repeats almost exactly the rail- 

 road section described above. 



Here gray and red shaly sandstones dip 25° E. beneath the tra}), and 

 .are distinctly baked for 2 feet and show a rust-filled columnar parting. 



The lower bed is 32 feet thick, with about 10 feet finely amygdaloidal 

 above and full of diabantite and calcite. The upper bed is massive and 44 

 feet is exposed. In it are four zones of crushing, 1 to 2 feet wide, with strike 

 N. 10° E., dip 70-80° W., which indicate small faults of unknown thnnv. 



Just above the mouth of the first brook entering Westfield Ri^-er from 

 the south, ea.st of Provens iMountain, in Agawam, a massive ledge of the trap 

 projects into the main stream, and the brook runs over the traj) in a pretty 



