276 N. S. SHALER BROAD VALLEYS OF THE CORDILLERAS 



natural protection, and, as the circles in question are often so distributed 

 that their margins are almost in contact, the result is that the sheets of 

 water which flow over the ground when a torrential rain, the so-called 

 " cloud-burst," occurs are likel}^ to find the earth quite without its nat- 

 ural protection against erosion. 



The only place where I have been abl^ to make any satisfactory ob- 

 servations as to the structure of the deposits of the detrital slopes of the 

 valleys at a depth of more than 30 feet below the surface is in the region 

 about Butte, Montana. At that point a number of shafts have been 

 sunk through the debris Avhich fills the valley of the Silver Bow river. 

 These shafts are all abandoned, so that the only information concerning 

 them has been derived from the waste about the pit mouths and from 

 the statements of those who remember something of what Avas encoun- 

 tered in the work of sinking them. It seems tolerably certain that the 

 deepest of those shafts sunk near the center of the valley slopes, which 

 are here about 2 miles wide, failed to attain the bed rock at a de[)th of 

 between 3 and 4 hundred feet, and that the material passed through 

 was principall}'^ of a nature approximating to arkose, with occasional 

 pebbles composed of the harder dikes which intersect the crystalline 

 rocks of the country. At other points in the valle3^s of the U{)per Mis- 

 souri system, esi)ecially in that of the Stinking Water, now renamed the 

 Ruby, borings which I had a chance to observe indicated that the ma- 

 terial to a depth of 50 feet and at a distance of about 4 miles from the 

 Old Baldy section of the Tobacco Root range contained coarse pebbles. 

 Various railway and irrigation ditch sections from northern Montana to 

 central Arizona clearly show that this pebbly nature is common to the 

 deposit. 



At some points this detritus forming the benches of the valley, when 

 it is of an arkose nature, has become cemented in so firm a manner that 

 it is readily mistaken for granite. In two instances I have known this 

 mistake made by competent mining experts, who were not to be per- 

 suaded of their error until they were shown that ordinary gravel lay be- 

 neath the firm beds. These recomposed crystalline rocks appear in all 

 cases to occur near the foot of the mountains' in places where tlie grains 

 of decayed granitic rocks have not been much rounded or deprived of 

 that part of their matter which was readily soluble. 



So far as the inadequate sections give information concerning the ma- 

 terials underlying the general surface of the valley, they indicate, as be- 

 fore noted, that the deposits consist of debris brought from the adjacent 

 mountains. This mixture of clay, sand, pebbles, and larger fragments is 

 rudely stratified ; the bedding is irregular and often scarcely traceable, but 

 has an evident dip toward the center of the valley in which the deposit 



