224 JAMES PERRIN SMITH 



In the Mother Lode region of the Sierra Nevada there are some- 

 what similar chert masses, in beds supposed to be Jurassic in age. 

 These too are probably of radiolarian origin. In the Middle Triassic 

 of Shasta County a series of siliceous shales almost without sand grains, 

 and about two thousand feet thick, likewise was probably formed 

 partly from the shells of siliceous organisms. 



The Lower Carboniferous and the Devonian of Shasta and Sis- 

 kiyou counties also contain many hundreds of feet of fine-grained 

 so-called siliceous shales that are probably, at least in part, meta- 

 morphosed organic sediments. Shells of diatoms and radiolaria are 

 extremely rare in all these older beds, but organic silica is very soluble, 

 and even a slight degree of metamorphism destroys the delicate tests, 

 and thus obliterates the evidence of their origin. 



Coal deposits. — During the Eocene epoch plant remains accumu- 

 lated to a considerable extent in the swamps of the old embayment 

 of California, especially along the western flank of the Sierra Nevada 

 near lone, the Coast Range island area of the Mt. Diablo region, 

 and in the middle Coast Ranges of Monterey, San Benito, and Fresno 

 counties. These leaf beds have since been compacted into lignite, 

 and in a few places into true coal. 



Chemical deposits. — In Kern, San Bernardino, San Diego, and 

 Inyo counties there are extensive chemical precipitates 'of salt, soda, 

 borax, and gypsum, concentrates from the old lakes and salt pans of 

 the arid region, from Tertiary up to the present. The areal extent 

 is not large, but they are scattered over enormous stretches of country, 

 and are of great present or prospective economic importance. 



Comparative Rate of Formation of Calcareous and 

 Arenaceous Sediments 



Most estimates of the relative rate of formation of calcareous and 

 arenaceous sediments are merely conjectural. A method is here 

 suggested by which a somewhat more reliable estimate may be made. 

 It is based on the comparative thickness of a single formation in the 

 Californian region with that of the same formation in another region. 

 This can be reliable only when the entire formation is represented in 

 both regions compared, and when the conditions are reversed. We 

 have two such cases in the Carboniferous of California and the western 



