240 Adventures in Scenery 



Gorgonio Pass; and the Palm Springs fault on the east, 25 miles. 

 The range is a vast uplifted block, tilted or rotated toward the 

 northwest on an axis crosswise of the length of the range. The 

 core of the mountain mass is granitic rock in the form of a 

 batholith, which was forced upward underneath and into very 

 ancient sedimentary formations of sandstone, shale, and lime- 

 stone, which are thought to be of Palaeozoic age or older. 

 These sediments are thought to have accumulated when the 

 region was a depressed area to a thickness of 10,000 feet or more. 

 These sediments were intruded by an igneous magma of molten 

 rock, and they now form several thousand feet of meta- 

 morphosed gneisses and schists. This is thought to have been 

 near the close of Jurassic time. The main body of the magma 

 of molten rock cooled slowly, resulting in the granitic batholith 

 which is now the core of the range. Erosion during a long 

 period has removed a great part of the overlying roof of sedi- 

 mentary metamorphosed rocks. The igneous material making 

 up the body of the San Jacinto Mountains is granite, and this 

 is the most widespread rock now exposed in the range. It 

 is best observed near Tahquitz Peak, where a large area is ex- 

 posed. About 200 square miles of such rock is exposed in the 

 higher mountains and an equal amount in the Sage Hills Up- 

 land to the southwest. Irregular areas of metamorphic rock, 

 sometimes in comparatively small patches, occur throughout 

 the mountains, remnants of the ancient sedimentary roof rocks 

 not yet removed by erosion. To the north the granite passes 

 beneath the alluvium of San Gorgonio Pass/ 1 " 



faulting and Upheaval of Geologically 

 Recent Occurrence 



Throughout a long time following the intrusion of the 

 molten magma, during which time erosion of the uplifted 

 sedimentary rocks was going on, so far as is known probably in 

 early Tertiary time, faulting occurred which later resulted in 



* D. M. Fraser. (See Appendix.) 



