4 University of California Puhlications in Botany [Vol. 9 



Canadian element in the boreal flora of the Sierra roughly coincides. 

 All of the region lies within the State of California save the Carson 

 Range just east of Lake Tahoe. 



PETROLOGY 



Both sedimentary and igneous rocks are included in that part of 

 the Sierra Nevada which is inhabited by the boreal flora. 



The sedimentaries are now reduced to isolated fragments of what 

 was once probably a continuous terrane beneath which the intrusive 

 magmas were irrupted. Most of the sediments date from paleozoic 

 and early mesozoic time and are now for the most part metamorphosed 

 to slates and schists. They are most abundant in the north, decreasing 

 in amount southward as the general height of the range rises, and 

 becoming restricted, in the high mountains, to the crests and summits 

 save at a few exceptional stations where they appear to have formed 

 massive blocks which sank in the still unconsolidated magma and have 

 so been preserved from the extreme disintegration to which similar 

 rocks at higher and more exposed situations have been subjected. 



Included in the sedimentary^ rocks are shales, sandstones, lime- 

 stones, and extensive areas of altered clastic rocks : schists,' slates, and 

 quartzites. The strike of the beds conforms to the trend of the range, 

 that is, from northwest to southeast, but the dip varies. While in the 

 Sierra as a whole the beds dip to the east, and generally at angles 

 between 40 and 90 degrees,^ in particular districts the dip is in 

 the opposite direction, as at Mineral King in the mountains west of 

 Mt. Whitnej^, where the beds dip to the southwest at an angle of 

 85 degrees.* The angle of dip has an important bearing on rock 

 weathering. 



The shales and their metamorphic products, slates and schists, 

 contain pyrite which stains the outcrops in tones of red and yellow 

 brown, causing them to stand out in contrast with the prevailing rock 

 tint given to the higher mountains by the light colored granitic rock. 

 In some places the slates are highly silicious, becoming converted to 

 cherty rocks. In the valley of Fallen Leaf Creek, west of Lake Tahoe, 

 there are exposures of dark colored banded silicious slates dipping 

 nearly vertically. On the east face of Mt. Tallac and farther west on 

 Jacks Peak, these same rocks appear. To the west and northwest of 

 Lake Tahoe similar rocks are exposed, changed in places to schists 



