no WARREN DUPRE SMITH AND EARL L. PACKARD 



lesser importance. The latest occurred after the extrusion of the 

 Columbia lava in the Miocene, followed by late Tertiary and 

 Quaternary concentrations as placers or secondary enrichments. 



Marine conditions were again initiated in later Knoxville time 

 by the advance of an arm of the sea along the northern flank of the 

 then partially dissected Klamath Mountains. The erosion pre- 

 ceding and continuing into this epoch had partially lowered those 

 mountains and had exposed rich quartz lodes to the actions of the 

 streams. Gold was thus washed out and deposited in the basal 

 gravels of the transgressing Cretaceous sea. By upper Knoxville 

 time the cold sea-waters of that time were cutting in the Franciscan 

 rocks near Roseburg and were possibly skirting the foothills of 

 the Blue Mountains. The meager boreal fauna of this sea gave 

 way to the abundant tropical life of the Horsetown Sea, which was 

 coextensive, in southwestern Oregon at least, with that of the pre- 

 ceding epoch. A rich Shasta flora upon the adjacent land gives 

 another glimpse of the forests of Oregon. 



In Chico time the sea, still teeming with tropical species of Indo- 

 Pacific affinities, entered the Rogue River basin and ultimately 

 reached the foothills of the Blue Mountains, possibly connecting 

 through the ancient Lassen Strait with a California sea forming the 

 Siskiyou Island of Condon. 



This marine epoch was brought to a close by the epeirogenic 

 and orogenic movements of the Laramide revolution, resulting in 

 the final withdrawal of the sea from eastern Oregon, the uplift 

 of the Klamath region, the folding and faulting of the strata, and 

 the intrusion by basic igneous rocks. It is not improbable that a 

 minor uplift along the axis of the present Cascades occurred at that 

 time, forming an effective barrier to the earlier Tertiary seas. A 

 long interval of erosion followed, which is in part represented in 

 California by the Martinez Eocene. During this time much of 

 the Cretaceous sediment was carried away, and southwestern 

 Oregon was reduced to a region of low relief before the next depres- 

 sion in western Oregon admitted a tropical Tejon Eocene sea over 

 western Oregon as far south as Roseburg and the mouth of the 

 Rogue River. Thousands of feet of silts and sands were deposited 

 in this shallow sea, which at times retreated locally, permitting the 



