S86 RALPH W. CHANEY 



In the second place the stratification where present is of the 

 lens and pocket type rather than in horizontal layers. This applies 

 to coarse as well as fine materials, but is especially conspicuous in 

 the case of the latter. Nearly all of the sandy and shaley phases 

 occur in the form of lenses or pockets having a horizontal distribu- 

 tion of a few tens of feet and a thickness, with few exceptions, of 

 not more than 15 to 20 feet. This is suggestive of fluctuating con- 

 ditions of deposition, due either to a variation in the kind of 

 materials available for transportation by the streams, to a variation 

 in the volume of the streams, or to a wandering of the streams over 

 the depositing portions of their courses. 



Both of these features suggest that the sedimentation was of 

 the bajada type, like that on the flanks of the Sierras.^ If the 

 Eagle Creek conglomerates were laid down as alluvial fan deposits, 

 it is clear from their thickness that they were deposited on the 

 flanks of a range of considerable height. This brings to considera- 

 tion a third feature, that of the variation of thickness in the area. 

 At no place is the base of the formation reached, but the observed 

 thickness at Red Bluffs is 2,700 feet, while south of the river its 

 exposed part is little more than 500 feet thick. This variation may 

 be explained in two ways: first, that it is due to the southward 

 plunge of the Cascade anticline, which would carry the top of the 

 formation down to within 500 feet of river level (assuming a dip 

 of about 10°); and, second, that it is related to the position of the 

 high land which was the source of the sediments, the deposits on 

 the immediate flanks being thickest, with a thinning outward 

 as the distance from the source increased. Assuming that the 

 Eagle Creek conglomerates were laid down on the flanks of an 

 east-w^est range, as shown by Fig. 4, there should be other evidences 

 of the proximity of the thicker section to the flanks of the range. 

 It is probable that this range contained the vents from which the 

 pyroclastics were ejected. We should therefore expect to find ash 

 and tuff more conspicuous in that portion of the bajada nearest 

 the range. At Red Bluffs, as has been stated, ash and tuff in 

 alternating layers comprise most of the visible portion of the Eagle 

 Creek formation, except near the top, where conglomerate is more 



' A. C. Trowbridge, Jour. Grol., XIX, 707-47. 



