GENERAL CHARACTERS, EXPOSURES, AND CONTACTS. 65 



basal layer more or less, the heat reached the water-soaked sand below, 

 and steam and mud frothed up into the mass of the still liquid lava in 

 great quantity. These abnormal conditions promoted the formation of 

 unusual varieties of trap. The absorption of water caused the formation 

 of much basic pitchstone, while repeated smothered explosions shattered 

 and commingled the heterogeneous products. 



GENERAL CHARACTER. 



For a thickness of 30 to 70 feet and for a distance of several miles in 

 the vicinity of Greenfield the basal portion of the trap sheet is a mix- 

 ture of sand, fragments of various sandstones, fragments of various kinds 

 of diabase, some with glass base, some with hyalopilitic base, and some 

 resembling andesites, all unlike the monotonous Triassic diabase, and 

 abundant fragments of glass, all cemented by glass, and variously shat- 

 tered and recemented, and the interstices filled by a water-deposited 

 mixture of albite, diopside, calcite, segirine-augite, and hematite. 



The main mass of the trap sheet is normal and continuous above this 

 confused mass, and in many places the basal portion of the sheet can be 

 seen to be a continuous mass of trap beneath the breccia, so that the 

 latter must have been formed in the midst of the sheet itself. The sheets 

 are normal, cotemporaneous sheets, often showing a ropy flow structure 

 at the surface. 



GREENFIELD QUARRY EXPOSURES AND CONTACTS. 



See plate 4. 



For a mile north of the quarry, beneath the observation tower,* east of 

 Greenfield, one can walk along the line of contact of the trap on the sand- 

 stone with the vertical wall of the trap rising above. Here there seems 

 to have been no distinct basal bed, but the whole mass was cooled nearly 

 to the crystallizing point when the sand rose at almost equal intervals, 

 and the streams of the sand and glass breccia formed by it rise in great 

 streaks or " schlieren," anastomose, and pass with fluidal structure around 

 the great rounded blocks of the normal trap which make somewhat more 

 than half the wall. 



At the quarry is a more distinct basal bed of trap 7 or 8 feet thick, 

 more or less shattered and displaced, and the sand can be seen continu- 

 ous with the underlying sandstones rising in rifts in this basal bed and 

 frothing out into a scoriaceous sandstone where it meets and blends with 

 the breccia above. This breccia is 60 feet thick — a greenish mass of 

 shattered glass and trap full of filaments of red sand shining with hema- 

 tite scales. 



* See Deerfield sheet of Massachusetts map. 



