CHARACTER OF THE MATERIALS FILLING THE CHANNEL. 77 
the consolidation of ashes and volcanic mud are usually called simply “ lava.” 
Other terms of local use will be noticed in the description of the various 
gravel mining districts farther on in this chapter. 
Among the non-voleanic portions of the mass there may be fine clays, 
very distinctly stratified, sands of every degree of texture, and gravels proper 
or collections of rolled pebbles, — the latter of all sizes and of various ma- 
terials. Some of the gravels are chiefly made up of quartzose pebbles ;. and, 
in places, boulders of this material form almost exclusively the mass of the 
gravel. The finely-laminated beds are known to the miners as pipe-clay ; 
and the terms sand and gravel are employed much as the geologist would use 
them. The gravel which is rich enough to be worked is called “ pay dirt,’ 
or “pay gravel,” and, if there is a well-defined bed of it, it is often termed 
the “lead,” or the “ pay lead.” If the gold is irregularly disseminated through 
the gravels, the term “spotted” is applied to them. 
In many localities the auriferous gravels with the associated sedimentary 
deposits having been entirely covered by volcanic overflows, either of solid 
lava or of detrital materials, this “capping,” as the miners term it, has never 
been removed by denudation, so that the underlying beds are entirely con- 
cealed, and there is sometimes a mass of unproductive rock several hundred 
feet in thickness piled over the gravel in which the gold occurs. By exam- 
ining the maps accompanying this volume it will be seen, in comparing the 
relative position of the gravel and volcanic deposits, how much of the former 
must be covered by the latter. The eruptive materials having come from 
high up in the range, their relative quantity naturally increases in that 
direction, as will be shown in detail farther on. The presence or absence of 
a capping of greater or less thickness of volcanic rock over the gravel is, of 
course, a matter of much practical importance to the miner. In the use of the 
hydraulic method of mining everything above the level of the bottom of the 
workings must be removed by the current of water, and must be of such a 
nature that it can be “cut” by the pipes, either as it naturally occurs or 
after being shaken up by the use of powder. The solid lava, however, can- 
not be brought into the sluices in this way, so that it is evident that a cap- 
ping of such material is a barrier to hydraulic mining. The less coherent 
kinds of lava can be washed down by the hydraulic jet; but material con- 
taining no gold cannot be handled without expense, so that it often becomes 
a question, in deciding upon the opening or the continued working of hy- 
draulic mining ground, how thick a mass of unproductive detrital material, 
