STRUCTURE OF THE VALLEY DEPOSITS . 277 



lies. Very often we find a mass of ordinary torrent waste intercalated 

 in the body of less transported material. These accumulations some- 

 times occur where they can not be certainly referred to any neighboring 

 torrent valley, yet they clearly are the product of some mountain stream 

 which has shifted its position since the pebbles were laid down. In the 

 depressions of the slopes through which the mountain streams pass on 

 their way to the main rivers I have found, notably in the valley of the 

 Ruby river of Montana, a class of materials in marked contrast with 

 those which make up the unconsolidated beds of the main valleys. Thus 

 in pits sunk to test the value of auriferous gravels at Alder creek, in Mad- 

 ison county, Montana, the deposits of a pebbly nature constituting the de- 

 trital cone of the stream which lies in a wide depression of the principal 

 slope of the vale have a thickness of from 20 to 60 feet, their depth in- 

 creasing toward the center of the valley. Beneath this pebbly bed, 

 with a very sharp transition, we come upon a grayish white clayey de- 

 posit of an absolutely homogeneous character, which has been bored 

 through to the depth of 90 feet without finding bottom. This clay-like 

 matter continues upward in the flat-bottomed gulch occupied by Alder 

 creek for about 5 miles from the mouth of the canyon ; at 3 miles above 

 that point it has a thickness of over 100 feet. 



Although occasional fossils are found in the torrent gravels which have 

 been formed since the Glacial period, they give no trustworthy evidence 

 as to the age of the deposits. I have gathered from Alder gulch, within 

 the upper limits of the detrital cone, near to or upon the foundation of 

 volcanic ash before referred to, several disjointed fragments of the skele- 

 ton of an elephant and teeth of Elephas 'primigenius^ but in the same sec- 

 tion have found a part of the jaw of a morosaurus. As there are evidently 

 no Cretaceous beds now existing in the valley of that creek, we may pre- 

 sume that this latter fossil came from deposits that have been completely 

 removed from the district. So far as I am aware, no fossils have been 

 found in the probably more ancient taluses of the broad valleys. 



As yet the nature and origin of the Alder Gulch clay is somewhat un- 

 certain. The components of the mass are so far deca^^ed that they can 

 not be clearly identified. The facts, however, warrant the supposition 

 that the mass was derived from a basic, volcanic ash. These facts are 

 as follows: The clay is quite unmixed with pebbles or even distinct 

 grains of sand ; it is not even stained with iron as an ordinary sedi- 

 mentary clay made in such a position would be. It exhibits no distinct 

 stratification. It does not become coarser on approaching the moun- 

 tain or even in the upper parts of the canyon of Alder creek. More- 

 over, on the westerly side of that stream, near the mouth of Granite 

 creek, it is immediately overlaid by lava flows which have certainly at 



