580 RALPH W. CHANEY 



feet at Red Bluffs on the Washington side to 500 feet on the Oregon 

 at Bonneville, a condition which appears to be due in part at least 

 to the southward plunge of the fold. 



The formation as exposed on Table Mountain and Red Bluffs 

 comprises a series of beds of tuff, ash, and volcanic conglomerate, 

 the conglomerate being most conspicuous near the top. In several 

 of the talus masses of the conglomerate poorly preserved leaf 

 impressions are found, and in both the conglomerate and the 

 tuff" sihcified wood is common. To the west of Red Bluffs cliffs of 

 conglomerate are conspicuous, and from them have slumped the 

 great masses of rock which have dammed the Columbia River, 

 resulting in its cascades. The base of the formation is reached 

 neither on the Washington nor on the Oregon side. 



On the Oregon side the maximum section exposed, 500 feet 

 thick near Bonneville, is a volcanic conglomerate, in most places 

 highly indurated. All the bowlders are of porphyritic basalt, some 

 of them reaching a diameter of 15 feet and averaging from one to 

 three feet in the coarser phases of the formation. The matrix is 

 a fine to coarse volcanic sand. Numerous pockets and lenses of 

 shale and sandstone are a characteristic feature. These are of 

 slight extent both vertically and horizontally, and in many cases 

 contain more or less well-preserved leaf impressions. Sihcified 

 logs and carbonized stems and fragments are of common occurrence, 

 representing driftwood deposited with the sediments. 



The frequence of volcanic activity during the deposition of the 

 sediments is indicated by the seams of volcanic ash which are seen 

 to overlie some of the soil layers representing old surfaces. A 

 quarter of a mile east of Bonneville occurs what is most probably 

 a contemporary extrusion of basalt, and on Greenleaf Creek the 

 sedimentaries are intruded by basalt. In these situations and else- 

 where locally the beds have been contorted and shattered, with the 

 development of slickensided surfaces and contact metamorphism. 



The upper surface of the Eagle Creek formation, overlain by 

 basalt flows, is markedly irregular, as first noted by LeConte in the 

 canyon of Tanner Creek.' Evidences of intraformational uncon- 

 formities are numerous and will be discussed below. 



' Op. cil. 



