rivsy^r:*" 



' 



^h | ™ HH^B ^ 



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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 volcanic 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. 131-137. 



