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TABLE MOUNTAIN: TUOLUMNE COUNTY. 



13° 



general idea of the position of the flow may be obtained. It breaks off pretty 

 abruptly close at Knight's Ferry, where the basaltic masses at its terminus 

 are very conspicuous. The total distance from the first crossing of the river 

 to the foot of the flow is about twenty miles in a straight line. The exact 

 amount of fall in this distance is not known ; but it appears to be on the 

 average about seventy-five feet to the mile. On the trail which crosses the 

 top of Table Mountain, on the way from Murphy's to Pine Log, the eleva- 

 tion of the summit, as determined barometrically, is 2,515 feet; southwest 

 of, and at the nearest point to, Columbia it is 2,247 feet; over the Buckeye 

 Tunnel west of Sonora 2,033 feet, which would give an average fall of about 



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eighty feet to the mile in this part of its course. 



Between Shaw's Flat and Montezuma, a distance of eight miles, the flow 

 of Table Mountain is very regular and uniform, and it is here that the 

 channel has been reached in quite a number of places by tunnels driven in 

 through the rim-rock, and mining has been carried on by drifting in gravel, 

 which here, however, forms but a narrow and thin belt. There is perhaps 

 no place in California where all the peculiar features of one of these lava 

 flows covering an ancient river channel are so well seen as here. To one 

 standing at a distance and looking in the direction of this flow, from a point 

 affording an unobstructed view of it, the horizontal upper edge of the dark 

 mass of lava, looking perfectly straight, but with an evident gentle descent 

 towards the valley, offers a marked contrast to the usual curving lines of 









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the eroded hillsides of the Sierra. The absence of vegetation on this table- 



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like elevation also helps to render it very conspicuous. It is seen at once 

 that the top of this flow is higher than the surrounding country, and that 

 there must have been a large amount of rock removed, by denudation, from 





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each side of it, in order that it should stand out above the adjacent country 

 in such isolation. It is evident that being, as is clearly seen from the nature 

 of the rock and its position, a flow of volcanic material down the slope of the 



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Sierra, it must have occupied a pre-existing depression or valley; by no 

 possibility can such a flow be conceived of as having followed the summit of 

 a ridge or spur. But there are other features which also furnish corrobora- 

 tive evidence as to the character of the surface when this erupted material 

 assumed its present position. Under the lava is found an extensive formation 

 of sedimentary origin, distinctly stratified, in a horizontal position, which could 

 only have been deposited from water standing still, or flowing with a very gen- 







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tie current. This stratified material varies in character ; much of it is a fine 



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