GENERAL DISTRIBUTION OF THE VOLCANIC FORMATIONS. 309 
Almost the only large gravel deposits which are not intimately connected 
with and very largely covered by volcanic materials are those extensive areas 
between Indiana Hill and Quaker Hill; and even here the igneous rocks are 
present in force in the immediate vicinity, and do extend themselves in part 
over the detrital beds. In fact, we find the gravel and volcanic masses some- 
times most intimately associated with each other; while, at other times, the 
connection of the two formations seems hardly anything more than an 
accidental juxtaposition. In the case of the Tuolumne Table Mountain, 
described in the preceding pages,* we have every feature marked with the 
utmost distinctness. The old channel of the river is there, and can be traced 
for many miles, perfectly continuous, in most respects closely resembling a 
river-bottom of the present time, but completely covered over and every- 
where concealed by a protecting mass of hard, indestructible, basaltic rock, 
which evidently once flowed as a lava current down the river valley. Were 
all the localities as simple in their nature as this, there would be little diffi- 
culty in making out the order of succession and the character of the different 
geological events which followed each other. In other regions, the position of 
the voleanic masses is apparently independent of that of the gravels; as is well 
illustrated in the neighborhood of Forest City (see Plate Q), where the chan- 
nel has a nearly north and south course, and the lava-flows cover all the 
spurs which descend from the summit between the southwesterly flowing 
streams. Still, on the whole, it is impossible not to admit the intimate asso- 
ciation of the aqueous and the igneous masses. This, in itself, is a matter of 
great interest, especially as we find that almost exactly the same conditions 
are repeated in another and far distant country, as will be set forth in a 
subsequent section, these two gold-bearing regions — California and Australia 
— being by far the most important ones in the world. 
The most obvious reason for the intimate association of the lava with 
the gravel is the simple mechanical one, that the former has acted as a pro- 
tecting cover for the latter, the volcanic materials having, by their inde- 
structible character or by their great thickness, either prevented the erosive 
agencies altogether from reaching the aqueous deposits beneath, or else 
greatly retarded their operations, so that portions at least of the under- 
lying material are left behind. This view of the case is, indeed, to a 
considerable extent the correct one. The heavy capping of lava has pre- 
served the gravels from being swept downwards, and has had a powerful 
* See ante, pp. 181-137. 
