420 J. E. MILLS ROCKS OF THE SIERRA NEVADA OF CALIFORNIA. 



of no reason for assuming that there ever were such in the region. The 

 conditions point rather to tilting of irregular blocks formed by approxi- 

 mately vertical or steeply sloping faults, and included within and sepa- 

 rated from the surrounding mass by fault planes. Such blocks have 

 been formed by Tertiary and Quaternary uplifting; indeed, the uplifting 

 has been by blocks, and each mountain is a block. Moreover, as a rule 

 the block is raised higher near one of the two longer edges (more com- 

 monly the eastern edge) than the other — that is, the block is somewhat 

 tilted. If the pre-Tertiary faulting was principally Mesozoic, and the 

 tilting of the blocks was carried farther than the Tertiary and Quater- 

 nary tilting until commensurate with the Mesozoic faulting, the presenl 

 structural conditions would result — that is. the strata would he thrown 

 on edge and those of any given block would he without connection by 

 arches or inverted arches with corresponding strata of adjoining blocks. 



The District more particularly described. 



The district in which my studies and surveys have been most detailed 

 lies between the eastern and western crests and between the North and 

 Middle forks of Feather river, and as my most definite illustrations are 

 from tins district I shall describe it briefly. 



The general topography and geology of the district are outlined on the 

 accompanying sketch map (plate 13).* Grizzly ridge, Hough mountain 

 and Green mountain form the eastern division of the range. Grizzly 

 ridge and Hough mountain rise on the northeastern side by steep escarp- 

 ment — a broken and jagged fault-plane — and on the southwest partlyby 

 steep escarpment and partly by slope, which however is steep. The 

 slopesand escarpments meet at the top in a sharp crest. At the westerly 

 side of the district rises Spanish peak mountain, which presents a very 

 steep escarpment eastward ; hut it- crest is the eastern edge of a plateau, 

 modified by erosion, some 13 to 14 miles wide. From the westerly edge 

 of this plateau the surface drops rapidly to the Great valley-of California. 



Between the two divisions of the range north of the Middle fork of 

 Feather river rises an intermediate mountain called Claremont. There 

 are also other ridges and mountains formed by uplift with axes of various 

 directions — one running nearly eastward from Spanish peak mountain 

 along the southern side of the East branch of the North fork of the 

 Feather, and one between Spanish creek and the Middle fork, formed by 

 a southwesterly uplift from Claremont, and a southeasterly one from 

 Spanish peak. Detailed surveys have proved that the topography, which 

 appeared a1 firsl sight to he the result of erosion and a simple system of 



Scale reduced from 1 inch I miles to 1 inch = C miles, or 1:380.060, in reproducing. 



