PREVALENCE OK FAULTING. 419 



have not been able to discover any unconformity of dip and strike be- 

 tween the strata of the pre-Mesozoic and Mesozoic groups : but the strata 

 of the two groups are unconformable by erosion. Those of the older 

 group were raised above sea-level and eroded, and then subsided to re- 

 ceive Mesozoic sediments. Moreover, they, or at least some of them, 

 were metamorphosed before the erosion, for pebbles and bowlders of pre- 

 Mesozoic quartzites as well as granites occur in Mesozoic conglomerates, 

 as will be hereafter shown. It is probable that the pre-Mesozoic uplift- 

 ing was, like the Tertiary and Quaternary uplifting, principally by fault- 

 ing, and therefore of little effect upon the prevailing dip and strike. 



Epoch of Tilting. — The upper Cretaceous (Chico) and Tertiary strata at 

 the western foot of the range dip westward at low angles. It follows, 

 therefore, thai the greater part of the tilting of the metamorphic rocks 

 took place before the later Cretaceous strata were deposited and after the 

 Mesozoic metamorphic rocks were deposited. According to Whitney's 

 determination, the Mesozoic tilting was done at the end of the Jurassic ; 

 according to Becker's apparently tentative and still incomplete deter- 

 mination, it was later or " post-Gault."* 



Character and Extent of Uplifting. — A part, at least, of the pre-Tertiary 

 uplifting was by fault, for on the easterly face of Claremont (see accom- 

 panying map, plate 13) pre-Mesozoic rocks are brought into contact with 

 the highest subgroup of Mesozoic strata. The displacement is in part 

 Tertiary and Quaternary- but the extent of this part can he measured 

 by the dislocation of Tertiary sediments and lavas. At one point at the 

 northern end of Claremont the vertical relative displacement of the Ter- 

 tiary materials is hut 1,300 feet, while the pre-Mesozoic slates there are 

 brought into contact with the thinly laminated shales of the upper part 

 of the upper .Mesozoic subgroup. The greatest relative vertical displace- 

 ment of Tertiary deposits at Spanish peak and Claremont is but about 

 ."..:'>oi) to 3,400 feet, while the shear of the fault is several times ;is great. 

 How much of the pre-Tertiary uplifting is due to pre-Mesozoic and how 

 much to Mesozoic movement I have found no means of testing. 



How the .Mesozoic uplifting and tilting was effected is not clear. Willi 

 the prevailing easterly dips, later rocks are often carried beneath older 

 ones; in other words, the strata have been overturned. The most ready 

 inference is that the strata were thrown into anticlinal and synclinal 

 folds by approximately horizontal thrusting, that these folds were over- 

 turned, and that during or after the folding the mass was faulted. But 

 the slopes of the range are steep, and over a large proportion of the area 

 tlic rocks are bare, and deep canyons afford numerous and extended ver- 

 tical sections ; yet neither arches nor inverted arches appear, and I know 



ill. Geol. Soc On vol. -'. 1890, pp. 201 



