324 RESUME AND THEORETICAL DISCUSSION. 
The immense size of some of the boulders in the gravel is a matter of 
interest, especially in view of the fact that they cannot, by any possibility, 
have been brought into their present positions by the agency of ice, which 
we well know to be capable of carrying the largest as well as the smallest 
of the fragments of rock which may be thrown down on to its surface from 
above. In some cases these enormous masses seem to be fragments from an 
adjacent vein, loosened from their original position by the decay of its walls, 
and thus allowed to slide down by gravity, the adjacent rock beneath having 
been eroded away in the ordinary manner. It is certain, however, that very 
large masses of rock can be transported by the agency of water alone. In- 
stances of this kind have been witnessed, not unfrequently in recent times, 
where the giving way of the barrier confining a lake or reservoir, or one 
of those extraordinary rainfalls popularly known as a “ cloud-burst,” has 
ulowed a large body of water to flow down a steep declivity.* 
The chemical changes which have taken place in the gravel since its 
deposition are more difficult to explain than the chiefly mechanical agencies 
which resulted in bringing together the great detrital masses under consider- 
ation. In the preceding chapters, as well as in the Appendices A. and B., 
there will be found numerous references to phenomena observed, which in- 
dicate more or less chemical action as having taken place, and as being even 
yet continued, in the various strata opened to examination by the hydraulic 
mining operations. These changes are displayed in many cases by the 
gravel itself as well as by the other detrital beds associated with the gravel 
proper, and also by the organic bodies imbedded in the formation. 
Decidedly the most important of these chemical reactions which present 
themselves for explanation in this connection are those indicated by the 
constant use of the terms “red” and “blue” gravel in the local descriptions 
given in the preceding pages. Indeed, the existence of these differences of 
color, which are accompanied by other practically important peculiarities, 
have sufficiently impressed themselves on the attention of the miner to lead 
to very curious theoretical views, to which reference has frequently been 
made in the course of the present volume. The famous ‘‘blue lead ”’ is the 
result of a desire on the part of the miners to connect together the frequent 
occurrences of dark-colored gravel, as being a special formation, differing in 
* It is stated —on good authority, as the writer believes —that during a cloud-burst in Mono County, Cali- 
fornia, a steam-boiler of considerable size was carried two miles down what but a few minutes before was a 
perfectly dry cation. 
