420 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



thick, which brancli iii)wnrd for about 7 feet from tlie main mass of the 

 sandstone and are full of small steam holes at the top'. 



A fine-graiued and finely porous, reddish trap is continued downward 

 from the compact trap above in all the interstices between the blocks, 

 cementing- them together in the same way that the sand below cements the 

 larger blocks, and the two cements meet along a horizontal line. (See PI. 

 VIII. The person seen in the plate stands on the sandstone and touches 

 this line with his finger.) 



As the great mass of lava flowed over the bottom of the liay, its con- 

 gealed and much fissured crust at the front of the flow, like an unrolling 

 carpet, gradually passed beneath the advancing mass, and the mud rose up 

 into all the fissures in the crust, while the heat baked it into a porous rock 

 and the still liquid lava within oozed into the cracks above to meet the mud. 



The above partial description of this most interesting locality was 

 made at a time when the f[uarry had exposed only a portion of the surface 

 to study.' A more careful examination of the place brought out these 

 facts: The basal portion of the bed is made up of angular blocks of trap, 

 and these blocks are often interlocked and a common structure passes from 

 block to block, showing that it is the portion of a bed of trap in place and 

 not a tuff or agglomerate of transported blocks. The blocks are of the 

 common, rather coarse-grained traj) of the sheet, but are distantly and 

 coai-sely vesicular, some of the spherical cavities being an inch across; and 

 what is most striking, many of the blocks have rows of these cavities ai-ound 

 their borders in whole or in part, and these cavities ai'e tubular at times and 

 closely set at right angles to the fissure which separates the block from its 

 neighbor. At times two adjacent blocks have a similar arrangement of 

 tubular cavities on either side of the crack. The arrangement of these tubes 

 at the surface of the blocks shows that the slow expansion of the steam was 

 effective after the mass had cracked into great blocks. Perhaps the increased 

 heat from its under-rolling and penetration by the liquid lava niav have been 

 effective here. Moreover, some of the blocks surrounded by the finer traj) 

 are quite spherical, as if they had been partly remelted after being envel- 

 oped in this newer trap. Again, it is a very partial description of the upper 

 portion of the wall to say that a finer trap has oozed down to meet the upcom- 

 ing red sand and cemented the blocks of trap. There is a well-defined line 



' See Am. Jour. Sci., 3d series, Vol. XLIII, 1892, p. 146. 



