50 GEOLOGY GOEGE OF THE COLUMBIA. 



The structure of the canon of Pit river, the relations which all the canons I have described 

 sustain to the basins drained through them, the sediments accumulated to such a depth above 

 the beds of the present draining streams, &c.. aside from the want of another adequate cause, 

 has led me to consider the gorge of the Columbia as formed, entirely or in part, by water. We 

 may obtain further evidence in the case by studying its 



LOCAL GEOLOGY. 



Within a few miles of the Dalles the river enters the &quot; gorge of the Columbia,&quot; and from 

 that time to its exit, fifty miles below, the view of the traveller is bounded on either side 

 by high mountains,, which rise with precipitous walls immediately from the water s edge. 

 Except in a few rare instances, where streams come down from the mountains, on either side, 

 there is no level land between the base of the cliffs and the river. This gorge lias everywhere the 

 appearance of having been cut through by the stream which now traverses it. In many places 

 there are perpendicular walls of trap rock many hundred feet in height, composed of different 

 strata, formed by distinct overflows, of which the cut edges are now exposed. These layers of 

 trap are often horizontal, and apparently continuous for miles. They frequently, too, exhibit a 

 columnar structure, the columns being perpendicular, and evidently have been subject to no 

 disturbance since their formation. Near the Cascades, however, there are evidences of very 

 recent volcanic action. The layers of trap are more or less disturbed, and the mountains, par 

 ticularly on the north side, exhibit large surfaces covered with blood-red scoria. 



Submerged forest. The river, from the Dalles to the Cascades, is very deep, has an imper 

 ceptible current, and has rather the appearance of an elongated lake than of a flowing stream. 

 At intervals, over the entire distance from the point where we entered the mountains to the 

 Cascades, the river is bordered on either side by the erect, but partially decayed, stumps of 

 trees, which project in considerable numbers above the surface of the water. This has been 

 termed the sunken forest, and has been generally attributed to slides from the sides of the 

 mountains, which have carried down into the bed of the stream the standing trees. This phe 

 nomenon is, however, dependent on a different cause. As I have mentioned, the vicinity of 

 the falls has been the scene of recent volcanic action. A consequence of this action has been 

 the precipitation of a portion of the wall bordering the stream into its bed. This impediment 

 acting as a dam, has raised the level of the water above the Cascades, giving to the stream its 

 lake-like appearance, and submerging a portion of the trees which lined its banks. Of these 

 trees, killed by the water, the stumps of many are still standing, and by their degree of pre 

 servation attest the modern date of the catastrophe. On examination, I found these stumps 

 to be the remains of trees of the Douglass spruce, which still forms the forests covering the 

 slopes of these mountains. 



Cascades. At the Cascades the river is deflected against the southern wall of its canon, and, 

 \ in a succession of rapids, falls sixty feet in three miles. 



The material which composes the dam in the river at the Cascades is a kind of conglomerate, 

 made up of fragments of trap rock, mingled with earth and sand. This is, in many places, 

 penetrated by threads of silica, which has often filled cavities and formed masses of agate and 

 chalcedony. In this conglomerate are imbedded many trunks of trees, which are sometimes 

 silicified, in other cases merely carbonized, and occasionally the same trunk exhibit both forms 

 of preservation. Of these silicified trunks there are some of large size, which so much 

 resemble recent wood as completely to deceive the eye. In many of them the structure 



