598 A TREATISE ON METAMORPHISM. 



shown" a high state of strain in the granite quarry at Monson, Mass. 

 Before the quarry was opened it would not have been supposed that the 

 rocks were subject to any unusual amount of stress, but when the quarry 

 was opened the rocks were found to be in such a state of strain that fre- 

 quent ruptures spontaneously took place, or occurred with very slight 

 assistance by the quarrying operations. . Before quarrying began the rocks 

 were prevented from lateral expansion, but after an excavation was made 

 they were freed from pressure on one side, and this loss of restraint led to 

 the rupturing already mentioned. That the Monson granite is elastically 

 compressed is shown by the fact that when a great mass is cut off from the 

 sides of the quarry it expands so that the bisected drill holes do not match. 

 Also elastic compression is shown by the expansion at various times during 

 the quarrying, anticlinal arches thus being formed by the elevation of lay- 

 ers along horizontal planes of rupture. Johnston had previously described'' 

 similar spontaneous movements in the sandstone strata at Portland, Conn. 

 Niles has also given evidence of very important strain in the sandstone at 

 Berea, Ohio, in the limestone of Lernont, 111., and in quarries in Connecticut, 

 At various places in the Galena limestone of the Fox River V alley, 

 Wisconsin, after excavation has been made, the layers have been found to 

 be under such elastic strain that they have released themselves by expan- 

 sion, forming anticlinal arches, in places 40 cm. in height. d Gilbert has 

 described e post-Glacial anticlinal arches at various places in the Devonian 

 shale of New York and northwestern Ohio and in the Trenton limestone of 

 New York. It thus appears that the rocks in the upper part of the belt of 

 cementation may be in a state of strain near or even to the elastic limit at 

 many places throughout extensive regions. The condition of mechanical 

 strain can be discovered only by quarrying processes, and by observations 

 such as are given above. When a specimen is taken from a quarry in which 



a Niles, W. H., Peculiar phenomena observed in quarrying: Proc. Boston Soc. Nat. Hist., vol. 14, 

 1871, pp. 1-8. 



h Johnston, John, Notice of some spontaneous movements occasionally observed in the sandstone 

 strata in one of the quarries at Portland, Conn.: Proc. Am. Assoc. Adv. Sci., 8th meeting, 1854, pp. 

 283-286. 



c Niles, W. H., The geological agency of lateral pressure exhibited by certain movements of rocks: 

 Proc. Boston Soc. Nat. Hist., vol. 18, 1876, pp. 272-284. 



''Cramer, Frank, On a recent rock flexure: Am. Jour. Sci., 3d ser., vol. 39, 1890, pp. 220-225. 

 Also, Cramer, Frank, On the rock fracture at the combined locks mill, Appleton, Wis.: ibid., 3d ser., 

 vol. 41, 1891, pp. 432-434. 



''Gilbert, G. K., Post-Glacial anticlinal ridges near Ripley and Caledonia, New York: Am. Geol., 

 vol. 8, 1891, pp. 230-231. 



