116 R. A. Daly — Mechanics of Igneous Intrusion. 



ate masses of solid rock now wholly or partially immersed in 

 the magma. 



The process is thus one of exfoliation on a large scale. It is 

 comparable to the rifting action of heat on masonry and on 

 granitic masses " said to have been made a matter of quarry 

 utility in India. It is stated (Nature, January 17, 1895) that 

 a wood fire built upon the surface of the granite ledge and 

 pushed slowly forward causes the stone to rift out in sheets 

 six inches or so in thickness, and of almost any desired super- 

 ficial area. Slabs 60x40 feet in area, varying, not more than 

 half an inch from a uniform thickness throughout, have been 

 thus obtained. In one instance mentioned, the surface passed 

 over by the line of fire was 460 feet, setting free an area of 

 stone of 740 square feet of an average thickness of five inches."* 

 Merrill gives a striking illustration of the exfoliation of granite 

 consequent on thermal expansion and general weathering of 

 the rock composing Stone Mt., Georgia.f Again, the observa- 

 tions made by Niles in connection with operations in several 

 I^ew England quarries corroborate the belief that the exfolia- 

 tion at plutonic igneous contacts must be well qualified to 

 shatter the invaded formations. At a gneiss quarry of Monson, 

 Mass., the rock is under notable stress due to local crustal com- 

 pression. A rough measure of the amount of that pressure is 

 found in the sudden visible expansion affecting great slabs of 

 the gneiss which, in the quarrying process, are freed from their 

 beds. "In one mass of rock, 354 feet long, 11 feet wide and 

 3 feet thick, the amount of expansion after dislodgment was \\ 

 inches up the slope of the hill. When the fracture by wedging 

 is suddenly and thoroughly made, the expansion takes place 

 immediately, and sometimes the expansive force itself completes 

 the desired work, the stone suddenly springing into the elon- 

 gated state. Spontaneous fractures also occur, of which one 

 was fully 4 inches wide. On removal of overlying beds, spon- 

 taneous upward bendings and swellings of the lower beds also 

 occur, most frequently in the thinner sheets, up to 4 feet in 

 thickness, with formation of miniature anticlinals. The amount 

 of elevation varies from \ inch to 3 or 4 inches, even in a 

 single afternoon. The span of the arch thus formed is some- 

 times 50 feet, while some are only 3 feet broad ; the crests 

 always trend in easterly and westerly directions, are sometimes 

 ruptured, and are evidently caused by expansive thrust in 

 northerly and southerly directions, since the edge of the sheet at 

 each base of the anticlinal arch remains so closely attached to the 

 underlying bed that no lateral slipping of this edge of the rock 

 could possibly have taken place.":}: Similar exfoliation and 

 arching of limestone is reported by Niles from Lemont, 111. 



*G. P. Merrill, Eocks, Rock-weathering and Soils, p. 182, New York, 1897. 

 f Ibid, p. 245. t J^ilien, op. cit., p. 386. 



