314 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



tend to separate the heated layer from the cooler portion 



underneath. He did not regard the presence of water as 



an essential feature. 



Becker, Branner and G. P. Merrill consider the domes as 



a product of weathering of homogeneous rocks. Becker^ 



writes : — 



III my opinion, the great granite domes are simply cases of exfoliation 

 similar, except in scale, to those often observed in basalt and other rocks, 

 and the regular curvature I believe to be due simply to the fact that, meas- 

 ured per unit of volume, the surface exposed is in inverse ratio to the radius of 

 curvature, so that the sharply curved surfaces weather fastest. 



Becker also points out that these curved surfaces often 

 cut the gneissic banding, showing exfoliation to be independ- 

 ent of the arrangement of the minerals in the rock-mass. 



Branner,'"^ whose observations were made largely in 

 Brazil, writes: — 



The unequal contraction and expansion of the minerals composing the 

 rock tend to disintegrate the entire mass, while the even annual and diurnal 

 changes and the approximately even penetrations of these changes cause the 

 rocks to exfoliate or to shell off in layers of even thickness like the coats of 

 an onion. 



MerrilF attributes exfoliation largely to temperature 

 changes, but considers the curved joints or partings in the 

 rock below the exfoliating surface* as " the result of tor- 

 sional strains and once existing are lines of weakness 

 which become more and more pronounced as weathering 

 progresses." Dana^ attributes concentric exfoliation of 

 many homogeneous rocks (granite, trap, sandstone, etc.) to 

 weathering and not to any original structure in the rocks. 

 Geikie*^ takes the same ground. 



In the Yosemite National Park, exfoHation is taking place 

 from nearly all massive granite surfaces. While best devel- 

 oped on the domes, excellent examples may be seen on bare 

 slopes, as that south of the top of the Nevada Falls, and 



1 Tenth Annual Report U. S. Geol. Surv., 1890, p. 142. 



2 Loc. cit., p. 281. 



3 Rocks, Rock-weathering, and Soils, 1897, pp. 180-184. 



4 Bull. Geol. Soc. Amer. Vol. VII, 1896, p. 245. 



5 Manual of Geology, 1895, p. 127. 



6 Text Book of Geology, 1893, p. 348. 



