THE BMD-ROCK SURFACE AND THE CHANNELS. 



333 





•surface water and the atmosphere. This is particularly well illustrated in the 

 case of metalliferous veins, which, almost without exception, tend to exhibit 

 (ewer indications of oxidized combination as greater depths are reached. 

 The lower portions of the gravel deposits are in this condition. Overlying, 

 as they do, regions of intense metamorphic action, preceding extensive vol- 

 canic manifestations in the adjacent region, they have been somewhat in the 

 condition of the matter enclosed within the walls of veins. The finer ma- 

 terials between the pebbles and boulders of the gravels have been cemented 

 together, partly by silicification and partly by reduction of the iron from the 

 oxidized condition to that of sulphuret. In this latter operation the re- 

 ducing agency of the organic substances enclosed in the formation has been, 

 in places at least, very conspicuous in its effects. Fragments of wood pene- 

 trated by and incrusted with pyrites are of common occurrence, and the 

 same is true to some extent of the bones and teeth of animals. Implements 

 and works of human hands have also been found, under the volcanic masses, 

 covered with a similar incrustation of sulphuret of iron. 



Section VI. — The Bed-rock Surface and the Channels. 



We have next to examine what is known in regard to the form and char- 

 acter of the bed-rock surface under the gravel deposits, for the purpose 

 of ascertaining what light this class of facts is able to throw on the physical 

 conditions prevailing during the period of the accumulation of the detrital 

 masses. In this connection we have two somewhat distinct kinds of phe- 

 nomena presented for our investigation : one of these relates to the form of 

 the surface of the bed-rock and the character of the markings upon it, as 

 indicative of the agency by which the erosion was performed ; the other has 

 to do with the connections of the channels or ancient river-courses with 

 each other, — a question involving, to a considerable extent, a, reconstruction 

 of the former topography of the whole region embraced within the field of our 

 investigations. This latter branch of the inquiry presents itself in a twofold 

 aspect, — one distinctly theoretical, the other eminently practical. The geolo- 

 gist, looking at the question from the scientific point of view, desires to 

 clearly establish the sequence of events which took place during the gravel 

 epoch, and to be able to assign for each of these a satisfactory cause ; the 

 practical miner, on the other hand, seeks to ascertain how the old channels 

 were connected, in order that he may be at as little expense as possible 



