338 MOUNTAIN RANGES 



escarpment, but with a tilted top that gradually slopes back to the 

 foot of the next block, to which it stands as the downthrow side. 

 The processes of denudation have carved these tilted blocks into 

 peaks and ridges of the ordinary kind. The boundary ranges, 

 the Sierra Nevada and the Wasatch, although mountains of fold- 

 ing, have themselves been modified by the same process, for 

 each of these ranges has a great fault along its base, the Great 

 Basin being on the downthrow side with reference to each of 

 them. The fault of the Sierras is on the east side of the range and 

 hades toward the east, that of the Wasatch is on the west side and 

 hades to the west. The dislocations of this region have been kept 

 up till a very recent period ; in southeastern Oregon the fault 

 scarps are still very plainly shown, the upthrow sides forming 

 ridges, and the downthrow sides valleys in many of which water 

 has gathered into lakes (see Fig. 104). Near Salt Lake City the 

 movements may even yet be in progress. In Fig. 52 (p. 132) is 

 shown an alluvial cone at the foot of the Wasatch Mountains ; on 

 the right side of the cone, near its upper end, may be seen a low 

 fault scarp, which could not have been very long maintained in 

 such incoherent materials. This exceptional mountain structure 

 seen in the Basin Ranges is, then, due to normal faulting. 



Another type of mountain structure different from either of 

 those mentioned is the laccolithic mountain. A laccolith (see 

 p. 283) is a rounded, intrusive mass of igneous rock which has 

 lifted up an arch of strata above it into a dome, but has not reached 

 the surface to flow out as lava. A laccolithic mountain may stand 

 isolated, like Little Sun Dance Hill (see Fig. 128), or several of 

 them may be grouped together, as in the Henry Mountains of 

 southern Utah, or again, they may form extensive portions of true 

 ranges, as in the Elk Mountains of Colorado. 



The Date of Mountain Ranges means the geological period in 

 which they were first upheaved above the sea. This date is sub- 

 sequent to the newest strata which are involved in the movement, 

 and earlier than that of the oldest strata which did not take part 

 in the movement, but must have done so, had they been present. 

 Strata which rest unconformably against the flanks of a range must 



