566 RALPH ARNOLD 
3. The cracks separating the scales from the dome-face always 
penetrate upward, never downward, and approximately parallel the 
outer surface of the scale. 
4. The incipient cracks are closely followed by a zone of weather- 
ing which often shows thin lamination parallel to the crack, and 
which, crumbling away, produces a crevice. The intensity of the 
weathering which goes on in this zone is often evidenced by the iron 
and other stains which emanate from the cracks and run down the 
surface of the dome. 
PREVIOUS THEORIES REGARDING THE ORIGIN OF DOMES 
Two general theories have been advanced in explanation of such 
peculiar structure, all heretofore, however, based upon observations 
of the development of domes in granite. Gilbert? states these theories 
thus: 
“According to one theory the separation of the granite into curved 
plates is an original structure, antedating the sculpture of the country 
and determining the peculiarities of form. According to the other 
theory the structure originated subsequently to the form, and was 
caused by some reaction from the surface.” 
Becker,” Branner,? Dana,* “Geikie,> (G. PR. Micmalll> Shaler 4 
Turner,® Gilbert,? and Chamberlin and Salisbury'® support the latter 
theory, while Le Conte,*? Muir,'? Whitney,‘ Bonney,’4 and some 
German writers'5 are in favor of the first. 
t Bull. Geol. Soc. Am., Vol. XV, 1904, pp. 29, 30. 
2U. S. Geological Survey, Monograph 13, pp. 70-72, 1888; Tenth Ann. Rept. 
U. S. Geol. Surv., 1890, p. 142; Bull. Geol. Soc. Am., Vol. II, 1891, p. 69. 
3 Bull. Geol. Soc. Am., Vol. VII, 1896, p. 281. 
4 Manual of Geology, 1895, p. 127. 
5 Textbook of Geology, 1893, p. 348. 
© Rocks, Rock-Weathering and Soils, 1897, pp. 180-184. 
7 Proc. Bost. Soc. Nat. Hist., Vol. XII, p. 289. 
8 Proc. Cal. Acad. Sci., 3rd ser., Geology, Vol. I, p. 315. 
9 Loc. cit., pp. 30-34. 
to “Processes and Their Results,” Geology, Vol. I, 1904, pp. 41-46. 
tr Elements of Geology, 4th ed., pp. 283, 284. 
12 Proc. Am. Assn. Adv. Sct., Vol. XXIII, pp. 61, 62. 
™3 “Geol. Surv. Calif.,”’ Geology, Vol. I, pp. 371, 372. 
14 Quart. Jour. Geol. Soc. London, Vol. XXXII, 1876, p. 149 
t5 Turner, H. W., loc. cit., p. 313. 
