424 GEOLOGY OF OLD HAMPSHIKE OOUiS'TY, MASS. 



this article, so tar as it relates to the Greenfield bed, at the expense of a 

 little repetition.^ 



The flow of the submarine lava bed seems here to have been unusually 

 rapid, and the under-rolling to have been a somewhat suliordinate phenom- 

 enon; still, the convection currents rising from the front of the bed seem to 

 have generally chilled it, so that a somewhat thin layer of compact, lieaAy, 

 iine-grained trap was soliditied and under-n )lled to form a basal bed protect- 

 ing the liquid mass above. When the sheet had advanced over the muddy 

 bottom so far that the imprisoned vapors could not escape laterally, some 

 slight and local disturbance broke up this liasal layer more or less, the 

 heat reached the water-soaked sand below, and steam and nmd frothed 

 up into the mass of the still licjuid lava in great quantity, carrying many 

 blocks of the basal bed. These abnormal conditions promoted the fonna- 

 tion of unusual varieties of trap. The pJjsorption 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 Grreenfield the basal portion of the trap sheet is a mixture 

 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 

 shattered 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 ;i contimious mass of traj) beneath the breccia, so that the latter must 

 have been formed in the midst of the sheet itself The sheet is a normal, 

 contemporaneous sheet, often showing a ropy flow structure at the surface. 



GREENFIELD ((I'ARRV EXPOSURES AND CONTACTS. 



For a mile noi'th of the quarrv beneath the observation tower east 

 of Cxreentield one can walk along the line of contact of the trap on the 



I Bull. Geol. Soc. America, Vol. VIII, p. 64. 



