408 University of California Publications in Geology [Vol. 13 



During upper Cambrian and Ordovician time the area of depo- 

 sition was far from land and the Furnace limestone was deposited. 

 In Silurian or Devonian time there was a general uplift followed by 

 continued sinking and deposition of sands, grits, conglomerates, and 

 other shallow water sediments. 



The next events of which we find any record were granitic intru- 

 sions of two periods, one possibly post-Carboniferous, the other Juras- 

 sic. The latter was accompanied or closely followed by mineralization 

 along many of the contacts between the igneous and the sedimentary 

 rocks. Some of the mineral deposits contained ores of gold, copper, 

 lead, zinc, and tungsten and will be described on another page. 

 Along with the granite intrusions were profound movements and 

 intense metamorphism of the involved rocks. 



There is no record of Cretaceous or early Tertiary sedimentation, 

 probably because erosion was taking place which removed the covering 

 from great areas of granite. It is barely possible that in Miocene 

 time the Potato sandstone was laid down. In Pliocene time, probably 

 in lower Pliocene, the Gulf of California extended northward into 

 the region of the present San Gorgonio Pass and sandstone was depos- 

 ited. By uplift this became isolated from the sea and the lower depres- 

 sions became playas. As the movement continued fanglomerate from 

 the neighboring hills covered the playa silts. The whole region was 

 reduced to a surface of low relief and basalt flowed out over the level 

 surface. 



In Quaternary time the country was subjected to stresses to which 

 the rocks yielded by faulting and a great block began to rise which 

 eventually became the San Bernardino Mountains. The uplift was 

 not continuous but by progressive stages with periods of rest, some of 

 which are still to be found recorded in the topography. Simulta- 

 neously the streams cut deep canons in the block and great beds of 

 fanglomerate, consisting of the transported debris, were deposited 

 along the flanks of the range. The mountains reached a great height 

 and, despite the fact that the general climate of the region was arid, 

 by their intercepting the moisture in the upper atmosphere they 

 received a precipitation equal to that of a humid climate. Glaciers 

 were even developed along the north side of the crest of the highest 

 ridge, but they were of short duration because of the increasing mild- 

 ness of the climate ; but even yet a little snow remains on the higher 

 slopes throughout the year. 



