64 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



force and were able to expand but slightly in the north-south direc- 

 tion because of the resistance of the adjacent rock. They have been 

 extended almost entirely in the vertical direction, but as this motion took 

 place against the great weight of the superincumbent rock, there was a 

 strong expansive stress or resistance in the north-south direction, and it is 

 this tendency to expansion, still stored in the gneiss, which would seem to 

 explain the sudden north-south elongation of blocks of the rock when they 

 are quarried. These most remarkable phenomena have been described 

 fullv by Professor Niles.' 



The face of the quarry looks ^\'esterly, and horizontal joint planes are 

 utilized in quarrying. Except for these planes the rock is remarkably free 

 from joints. Slabs 3 to 5 feet in thickness and 10 feet wide from east to west, 

 and of very great length from north to south, are split off by a long line of 

 wedges, and while one end of the rock still retains its connection with the 

 ledge the other expands so that the halves of the drill holes fail to match. 

 In one case, in 1869, a block 4 feet thick, 11 feet wide, and 354 feet long 

 was split bv the use of nearly 1,200 wedges. As the block was followed 

 up from the attached end the halves of the di-ill holes soon ceased to match 

 exactly, and this increased with regularity to the other end, where the elonga- 

 tion amounted to an inch and a half. Many such cases have occurred at all 

 seasons and times of the day. Several were carefully studied by Professor 

 Niles, and I have myself seen one most striking case. Where a long line 

 of wedges was put in about 6 feet back from the quarry face, and before 

 the cross channel was cut at the south end of the proposed block, the crack 

 started of itself and ran beyond the line of the wedges for a long distance 

 to the north, while at the south end it soon left the line of the wedges and 

 went west, and ran out to the (juan-y face, and the expansion then caused 

 the block to project at the south end westerly over the face of the quarry. 

 As much as 10,000 tons of rock have been quarried out by a single fissure. 



In the same way the expansion causes the horizontal sheets of the rock 

 to rise, often quite suddenly, in considerable anticlines, with the arch as much 

 as 50 feet long and the rise 3 or 4 inches. These anticlines form some- 

 times with explosive violence, throwing large fragments of the rock more 

 than 2 feet from their original position. The large area of shattered rock 

 produces the impression of a small but violent earthquake. The explosions 



iProc. Boston Soc. Nat. Hist., Vol. XIV, p. 80; Vol. XVI, p. 41; Vol. XVIII, p. 272; and Proc. Am. 

 Assoc. Adv. Sci., Vol. XXII, part 2, p. 156. 



