THE COAST RANGES A MOUNTAIN SYSTEM. 101 



forces did not always act upon the already existing axes, but new ones 

 were formed by their sides or even in divergent directions. 



The Coast Ranges Constitute a Mountain System. 



Professor J. Le Conte gives the following definition of a mountain 

 system : 



"A mountain range is a single mountain individual, born at one time (mono- 

 genetic) — I. €., the result of one, though it may be a prolonged, earth effort— as 

 contradistinguished on the one hand from a mountain system, which is a family 

 of mountain ranges born at different times (polygenetic) in the same general region, 

 and on the other from ridges and peaks, which are subordinate parts, limbs and 

 organs, of such a mountain individual." 



It seems to the writer that this definition is applicable to the Coast 

 ranges ; that they should properly be considered a system of mountains, 

 embracing, as they do, ranges of such different ages. This is particularly 

 applicable to the region south of San Francisco bay. On the north there 

 is not shown such a complexity of structure. 



If the writer's views are correct, the Coast ranges are not younger than 

 the Sierras, as has been generall}^ supposed. According to our present 

 information, land areas existed in this region before the Jurassic up- 

 heaval, which, in the opinion of Dr Le Conte, gave rise to the main por- 

 tion of the Sierra Nevada. 



Sketch of the Geologic History of the Coast Ranges. 



The age of the crystalline schists and limestone of the Coast ranges is 

 unknown. At some period, probably during the Paleozoic or possibly 

 earlier, an upheaval accompanied by the formation of granite took place 

 along the axis of the Coast and Peninsula ranges, intensely metamorphos- 

 ing the sedimentary strata. Owing to the fact that no trace of uncrys- 

 talline rocks older than the Jurassic has been detected along this axis, 

 it seems justifiable to suppose that during the early Mesozoic, and per- 

 haps later Paleozoic, the land area of the crystalline rocks was far more 

 elevated and extensive than at present. Erosion through long intervals 

 of geologic time would be necessary to remove all traces of the less 

 crystalline upper rocks of this complex. 



If authorities are correct, a broad sea stretched to the east over the 

 most of the Sierra region during the period of erosion of this crystalline 

 axis of the Coast ranges. As Mesozoic time progressed a subsidence 

 began and continued through the Jurassic, with the deposition of wliat 

 has been termed the pre-Cretaceous series. At the close of this period 

 the great revolution in the Sierra Nevada took place, accompanied l)y a 

 tilting and folding back of the strata and the formation of an enormous 

 area of a fused granite magma. At the same time an axis of uplift was 

 formed in the Coast ranges, being connected with the Sierra Nevada at 



