ABSENCE OF CRETACEOUS DEPOSITS. 383 



great lapse of time, the records of which arc to be found to the westward 

 in the deposits bordering chiefly upon the Sacramento valley. 



That there was in the northern Sierra Nevada, region an epoch of great 

 disturbance after the deposition of the Jurassic rocks near Taylorville is 

 clearly shown by the fact that those rocks are overturned and faulted.' 

 That the disturbance and elevation occurred immediately at the close 

 of the Jura is rendered highly probable by the complete absence from 

 the Taylorville section of any Cretaceous deposits. It is possible that 

 the Cretaceous, if formerly present in that region, has been completely 

 removed by the great erosion to which the Sierra Nevada bus been long- 

 ex posed : but of this view I have not been able to obtain any supporting 

 evidence. 



So far as yet known, on the fortieth parallel the rocks next younger 

 than the Taylorville Jurassic are the Knoxville beds of the earlier Cre- 

 taceous. They are widely separated in space, and it is probable that 

 there was between their periods of deposition a considerable lapse of 

 time, within which the rocks of the Sierras were greatly deformed I >y <•< im- 

 pression and raised above the sea; consecmently the shore-line of the 

 Cretaceous sea scarcely reached the western base of the Sierra Nevada 

 and laid down its deposits unconformably upon the older rocks. f 



In order fully to comprehend what is represented by the Neocene- 

 Jura unconformity of the Taylorville region it is necessary to consider 

 the relation of the Johnson gravel to the Cretaceous rocks of the Sacra- 

 mento valley. These gravels were deposited by a stream flowing into the 

 Mountain meadows region, where some Miocene plant remains have been 

 found. It has been shown I that these sandstone and gravel strata 

 probably conned beneath the lavas of Lassen peak with deposits of the 

 same age on Little Cow creek, at the northeast corner of the Sacramento 

 valley. At this last locality the Miocene strata resl unconformably on 



the ( !hiC0 beds of the ( 'I'etaceoiis. 



Iii the Neocene-Jura unconformity, therefore, we have represented not 



only the great time interval between the close of the -Lira and the Mio- 

 cene, hut also two unconformities, the first, and by far the most conspic- 

 uous, between the Cretaceous and the Jura and the second between the 

 ( Iretaceoua and the Miocene. 



Pleistocene-Neocene Unconformity. — The valley alluvium (Pleistocene) 

 docs not come in contact with the Johnson gravel, and yet their uncon- 

 formity, du«' to erosion, is will marked. The valley alluvium was de- 



* Hull. Geol. Soc. Am., vol, •_'. p. 206. 



fThat the ks of the Sh > iwn rest unconformably ii| the older rocks is 



well known. Our knowledge of this unconformity has recently been much extended bj Mr. II W. 

 Fairbanks in the imi riean Geologist for March, 1892 (vol. ix, pp. 153 I HO 



t Eighth \iui. Rept. U. S. Geol. Survey, pt. i, pp. 1 1 > 122. 



