

PECULIARITIES OF THE GRAVELS. 



323 





Among the occurrences which arrest the attention of the investigator in 

 the hydraulic mining region there are two which seem very noteworthy. 

 One is, the fact that in some localities the gravel is almost entirely made 

 up of quartz boulders and pebbles ; the other, that some of the boulders are 

 of such enormous and quite exceptional size. Each of these peculiarities 

 demands a few words of comment. 



The gravels, of course, consist essentially of the hardest portions of the 

 rock masses, which have been eroded away during the time of their accumu- 

 lation. A large part of the metamorphic crystalline schists of the auriferous 

 belt are of a decidedly indestructible character. Hence the gravel deposits, 

 as a general rule, do contain a very considerable proportion of this kind of 

 rock, as has been mentioned so frequently in the preceding pages ; but 

 there are districts where the bed-rock is chiefly made up of finely laminated 

 slates, which are sometimes very soft, or are easily rendered so by exposure 

 to air and moisture. Such softer rocks are not un frequently traversed by 

 large quartz veins ; and, in some cases, the slates adjacent to the veins seem 

 to have undergone some chemical change, rendering them peculiarly liable 

 to disintegration. It follows, therefore, that in the wearing away by water 

 of a region where the bed-rock is of this character, it would be the natural 

 result that the softer material should become almost entirely pulverized, and 

 the resulting mud be carried oil 7 to a considerable distance, leaving behind 



only the indestructible portion, or the quartz. We know that, at the present 

 time, the masses of quartz enclosed in the slates are sometimes of great width, 

 occasionally exceeding a hundred feet. The breaking up of such masses 

 would be much assisted by the occasional softer streaks of slaty rock which 

 they are liable to contain, and of these only faint traces might remain after 

 the abrasion, removal, and redeposition of the quartzose material had been 

 effected. 



There may have been localities, however, where the silicious deposit spread 

 itself extensively over the entire surface, so that large masses were accumu- 

 lated, quite free from any admixture of other rock. Something of this kind 

 may be seen at the present time in process of formation at the well-known 

 locality of Steamboat Springs, in Nevada, The breaking up and washing 

 away of this enormous deposit might naturally give rise to an accumulation, 

 somewhere along the line of direction in which it was carried, of quartz- 

 ose material, quite free from any admixture of slaty or other kinds of 

 Metamorphic rock. 



