The Shoshone Island. 75 



arated the waters east of it from those west- 

 ward, the island waters to the eastward still 

 encircling their island, the Shoshone, as those 

 of the west continued to encircle the Siskiyou 

 with marine environment. The inland waters 

 slowly changed to brackish, then to fresh 

 water. But these inland waters were very 

 extensive. They received from the east the 

 whole watershed of the western slope of the 

 recently elevated Rocky Mountain range. 



The discharge oceanward of this great 

 watershed slowly divided into three areas of 

 drainage, the southern one laying the foun- 

 dation of the drainage of the Colorado river, 

 the northern one laying the foundation of the 

 drainage of the Columbia river, while the mid- 

 dle region awaited the slower process of drain- 

 age by evaporation. 



The double results of these elevations of 

 surface and erosions of drainage currents 

 slowly subdivided the enclosed waters, until 

 the interior of Oregon, to which region our 

 story is restricted, became covered with ex- 

 tensive lakes connected by drainage links 



