480 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



SOURCE OF THE MATERIAL OF THE TUFF BED. 



The sections of the diabase from the tuff show varieties containing (1) 

 much glass base, (2) well-developed porphyritic augites and olivines, (3) 

 development of augite as early as the oldest feldspars, (4) inclusions of 

 quartz grains, all peculiarities common in the newer dikes and not found 

 in the older diabase of the Mount Holyoke bed. The newer dikes lie 

 along the same line with the tuft', and are punched up through it. 



The description of the base of the bed given above indicates that an 

 explosive eruption of diabase occurred somewhere along this line, which 

 furnished the great mass of material whose length of outcrop is 10 miles 

 and whose thickness is about 550 feet. The later dikes seem to have been 

 driven up through this tuff" bed, and, I think, lie along the line of the 

 great fissure up through which the diabase of the Holyoke-Mount Tom 

 bed comes. 



It is just south of the ruined stone mill above Smiths Ferrv that the 

 tuft' bed is thickest and the blocks in it are largest, and it contains many 

 fragments 1 to 3 feet across. Their size diminislies southward; 1 mile south 

 (R. Houston's) the fragments reach 8 inches; at 2 miles (P. Brenu's) 4 inches, 

 and here the tuft" has dwindled to three beds 3 feet thick, and the mass of 

 the fragments ai'e about an inch long, though above the upper bed distant 

 rounded fragments 5 to 6 inches long appear in the sandstone. The place of 

 eruption seems to have been somewhat, but not far, north of Smiths Fen-y. 

 It is interesting to trace the graphite found in the tuff" and in the sandstone 

 above it to its source, which must have been in the region of Sturbridge, 18 

 miles to the east, while the crystalline boundary on the west is only 9 miles. 



HOLLOW BOMB FROM DELANEy's QUARRY, NORTHAMPTOX. 



I dislodged from the tuff' at this qxiarry a rounded bomb 1^ by 1 by 1 

 inch, A\'ith its center coarsely amygdaloidal, the cavities of such shape that 

 they could have been formed only by steam and not by later weathering, 

 and a border from ^ to ^ inch wide which was completely compact. 



PETROGRAPHICAL DESCRIPTION. 



The rock from large blocks in tuff from roadside south of Smiths 

 Fen-y, where the railroad makes a deep cut in the tuff, is a typical diabase 

 with decomposed feldspars; a brown glass, generally devitrified and filled 



