Geology 



On consulting the barometer we found that these 

 terraces were approximately on the respective levels of 

 the three dry overflow channels which we had previously 

 observed on the spur of the hill which formed the 

 southern boundary of the valley (see page 260), and we 

 concluded that they had been formed as deltas in the 

 glacier lake which had formerly occupied this site, each 

 delta corresponding to a former water-level determined 

 by the heights of the successive overflows. 



At the mouth of the gorge cut by the stream in the 

 lower terrace was a great fan of gravel, the result of the 

 erosion of the terrace gravels. In this we found large 

 and workable quantities of tin-stone (SnO 2 ) concentrated 

 by the action of the stream in a manner similar to that 

 in which the gold of Three Forks and the magnetite of 

 East Valley had been accumulated. 



Tin-stone is very hard and durable and is also con- 

 siderably heavier than quartz and felspar, the other 

 constituents of the deposits. Its specific gravity is about 

 7, while that of quartz is only 2*65 and that of felspar 

 from 2*5 to 27. The ore occurred as small dark brown 

 rounded grains, varying in size from that of a pea down- 

 wards, and was often to be seen in the gravel-fan con- 

 stituting beds several inches in thickness. The whole 

 deposit, however, covered less than half a square mile, 

 and it appeared doubtful whether it would at present 

 pay to work it, owing to the remoteness and inaccessi- 

 bility of the district. The terrace gravels, which were 

 much more extensive, also contained some tin-stone, but 

 were not nearly so rich as the material of the fan. 



All this tin-stone must have been derived from the 

 rocks of the upper part of the valley, and we consequently 



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