348 



SCIENCE 



[N. S. Vol. XXX. No. 767 



But the upper Knoxville beds, while still re- 

 taining reminiscences of the Boreal Eegion 

 in Aucella and a few other forms, show a 

 preponderance of life characteristic of more 

 favorable conditions. Aucellas of northerly- 

 habit mingle with cephalopoda that did not 

 belong in the Boreal Eegion, and on the near- 

 by land cycads abounded. The line between 

 Jurassic and Cretaceous should be drawn, not 

 at the beginning of the Knoxville, but be- 

 tween the lower and the upper Knoxville beds ; 

 the former belonging to the Portland and 

 Aquilonian, while the latter belong to the 

 Neocomian. 



With the opening of the Horsetown epoch, 

 the revolution of faunas and floras was com- 

 plete, the climate had become tropical, and 

 swarms of Trigonias, Nautilus and Ammo- 

 nites like those of India and eastern Africa 

 occupied the shallow seas of northern Cali- 

 fornia. These beds were deposited only in a 

 narow strip from Shasta County down to the 

 neighborhood of Mt. Diablo, the rest of the 

 state being above water. 



While the Paleozoic and the earlier part of 

 the Mesozoic were characterized by the forma- 

 tion of immense masses of limestone, and the 

 Jurassic and the Knoxville by the deposition 

 of thick beds of shale, the middle Cretaceous 

 inaugurated a sandstone-forming era, which 

 lasted through the entire Tertiary. 



During the Upper Cretaceous Chico epoch 

 the climatic conditions and faunal geography 

 remained unchanged, but the sea encroached 

 still farther on the land, reaching the foot of 

 the Sierra Nevada, where, in Butte County, 

 the unaltered and slightly tilted sandstones 

 of the Upper Cretaceous may be seen resting 

 upon the upturned, metamorphosed and 

 eroded rocks of the backbone of California. 



By the end of Cretaceous time the sub- 

 sidence and erosion of the western part of 

 the continent had almost established a con- 

 nection between the Pacific Gulf in California 

 and Oregon and the old Mediterranean Sea 

 of the Mississippi Valley. The intervening 

 isthmus not covered by salt water was worn 

 down to base-level, and wide expanses of flats 



were covered with marshes, which eventually 

 formed coal, preserving a very similar flora 

 from the outliers of the Mississippi Valley 

 almost to the Pacific coast. These coal-form- 

 ing conditions reached far up into Alaska, 

 where almost under the Arctic Circle types of 

 plants flourished that, to-day, could not live 

 in the open north of Mexico. 



In Eocene time the climatic and geographic 

 conditions remained the same as in the Upper 

 Cretaceous, but the sea had encroached still 

 farther on the land, and the base-leveling of 

 the backbone of the continent was more com- 

 plete. The aged rivers began to deposit their 

 loads of sediments, beginning the formation of 

 the auriferous gravels, the first great source 

 of wealth of the Pacific coast. 



Tropical conditions still prevailed up as 

 far as Alaska, and coal was still formed 

 abundantly where vegetation is now scanty. 

 If a geologist in western America had first 

 named the geologic systems, the Eocene would 

 have received the name " Carboniferous," for 

 most of the coal on the west coast belongs to 

 that epoch. During the Eocene, also, a 

 temporary connection was established between 

 the Pacific and the Atlantic basins, for in 

 California and Oregon the Atlantic " finger- 

 post of the Eocene," Venericardia planicosta, 

 is found along with Pacific types. 



Before the Miocene epoch this Atlantic con- 

 nection had ceased, and the faunas of the 

 later Tertiary were wholly of the Pacific type. 

 The lower Miocene was still warm, for we 

 find in its fauna a Nautilus still persisting, 

 and other genera now found only in southern 

 waters. Quiet accumulation of sediments 

 with abundant organic remains, diatoms and 

 radiolaria, was going on in the Coast Range 

 region. From these the petroleum, which has 

 added so much to the wealth of California, 

 was afterwards distilled, in the great disturb- 

 ance that took place after the close of the 

 Monterey epoch of the middle Miocene. 



The vast outpouring of the Columbian lava 

 flow, which covered an area of more than two 

 hundred thousand square miles, including the 

 northeastern part of California, occurred 



