232 THE PHYSICAL MANIFESTATIONS OF WEATHERING- 



As above znentioned, the sheets of granite, varying from a few 

 inches to several feet in thickness, conform in a general way to 

 the present surface of the hill. Constant expansion and con- 

 traction from temperature changes have, in the manner already 

 described, so expanded these sheets that, bound at the sides, 

 they have found relief in an upward direction where resistance 

 was least, and risen in dome or roof shaped forms, as shown in 

 the sketch. (Pig. 18.) The weight of the sheets higher up the 

 slopes, impinging upon the edges of those below, has in some 

 cases undoubtedly aided in the work, but the larger part is due 

 to simple expansion, such as was referred to on p. 159. 



These ruptured sheets are rarely more than 10 inches thick, 

 but 10 or 20 feet in diameter. The material, though quite fresh 

 appearing, is loosely granular and friable, easily reduced to sand. 

 This same mass of granite sometimes shows upon its surface 

 peculiar circular depressions, one within another, separated by 

 intervening ridges of low relief, such as have been described in 



a much more perfect 

 stage of development by 

 Dr. Robert BelP in the 

 ITuronian rocks of Can- 

 ada. These, as shown in 

 Fig. 19 from Bell's paper, 

 are some 3 or 4 feet in 

 diameter and 3 or 4 inches 

 high. The cause of this 

 form of weathering at 

 Stone Mountain is not ap- 

 parent, though Bell, in the 

 case figured, regards it as 

 induced by an original 

 concretionary structure. 

 '^&^M^^0^:^W^'¥W^ The spheroidal struc- 

 l^iG. 19. ture so frequently seen 



in basaltic rocks, and 

 as typified in the spJiaeroidische ahsonderung of German writers, 

 may perhaps be due to an original spheroidal tendency caused 

 by cooling,^ but a very large proportion of, the spheroidal masses 



^Bull. Geo! Soc. of America, Yd. Y, 1894, p. 362. 

 2T. Q. Bonney, Quar. Jour. Geol. Soc. of London, Yd. XXXII, 1876, p, 

 153. 



