Geol.-Vol. I.] TURNER-ORIGIN OF YOSEMITE VALLEY. 313 



that the granite is flaking off in huge scales, which nearly 

 but not exactly conform in shape to the curved sur- 

 face of the dome. As stated by Branner,' these scales 

 overlap like shingles, so that the projection upward of the 

 inner curve of each scale would carry it into the mass of 

 the dome, a fact brought out better by an exfoliating sur- 

 face near a little lake, known as Royal Arch Lake, that 

 Hes northeast of Johnson Lake, in the southeastern por- 

 tion of the Yosemite quadrangle. The same thing jnay be 

 seen at the Royal Arches, and on North Dome, in Yo- 

 semite Valley. 



Whitney thought that exfoliation was due to an original 

 concentric structure in the granite, formed on cooling. 

 Bonney and some German writers adopt the same explana- 

 tion. Concentric structures certainly exist in what are 

 known as orbicular rocks, where there is a nucleus around 

 which different minerals are deposited in concentric layers, 

 but so far as can be determined from an inspection of the 

 rocks of which the domes are formed, they are usually 

 quite Homogeneous in texture, and where they are gneissic 

 or banded the concentric exfoliating shells are as likely to 

 cut across the banding as to conform to it. Another struct- 

 ure seen in some lavas ^ is an original spheroidal structure 

 which may have resulted from flow movements while the 

 lava was quite viscous. In this case the spheroids do not 

 exhibit an orbicular or concentric structure due to the 

 arrangement of the minerals. 



Whitney, Bonney', and others, however, do not appear 

 to have in mind either of the structures referred to above, 

 but rather a physical structure due to contraction on cool- 

 ing, perhaps like the perlitic structure of glassy lavas. 



In 1869, Shaler^ called attention to the concentric weath- 

 ering of granitic rocks and to its superficial nature. Shaler 

 considered that the shells were formed by the heating and 

 consequent expansion of the outer layer, for this would 



1 Bull. Geol. Soc, America, Vol. VII, 1896, p. 270. 



" Ransome, Bull. 89, U. S. Geol. Surv., 1898, p. 23. 



3 Quart. Journ., Geol. Soc. London. Vol. XXXII, 1876, p. 149. 



< Proc. Bost. Soc. Nat. Hist., Vol. XII, p. 289. 



