350 GEOLOGICAL SURVEY OF THE TERRITORIES. 



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perhaps in character to the Wealden formation of Europe, that is placed 

 between the Jurassic and Cretaceous. Views of the present day regard- 

 ing "formation " and " age " are merely conventional, and it is more than 

 probable that by the time a little more of the remaining four-filths of 

 the land on the surface of the earth are explored, geoguostically and. 

 geologically^, thej^ will be compelled to undergo considerable modifica- 

 tion. 



Tertiary, Diluvial, and Alluvial deposits are found under such condi- 

 tions as would be favorable to their formation. In consequence of 

 the rise of the main body of land, the marine Tertiary was forced to 

 retreat farther toward those portions that were still submerged, and no 

 strata were found that could be referred to this epoch unless of a local 

 fresh-water character. Drift, belonging both to Diluvial and Alluvial 

 ages, is abundant. Alluvial deposits, as everywhere, owe their charac- 

 ter to the rock to which they owe their existence, and the old principle 

 of " same cause, same efl'ect " is clearly discernible. 



Glacial phenomena, that seem to be more abundant farther north, were 

 recognized in but one locality, and there they were not of any consid- 

 erable extent. 



By far more varied, and of deep interest, are the volcanic eruptive 

 rocks, so widely distributed throughout the district. Eichthofen's 

 excellent classification, comprehensive and yet concise, finds a new 

 support in the twenty-three hundred square miles of trachorheitic 

 material contained in our district. Throughout, the given character of 

 andesite, trachyte, and rhyolite could be relied upon as soon as the posi- 

 tion of andesitic tuffs was properly recognized ; and although the dif- 

 ferent conditions undoubtedly influencing the cooling of the masses at 

 different points ijroduced almost innumerable varieties, even within 

 limited areas, his appreciation of identity and correlation was not im- 

 X)aired. A question of great importance arises during the contempla- 

 tion of the immense quantities of this material ; it is the one touching 

 their origin. Without entering upon the various hypotheses that have 

 been advanced upon this point, with a semblance of more or less prob- 

 abihty, I will make a few suggestions that have occurred to me as the 

 necessary consequence of the observation of facts. 

 As stated above, it was impossible to find even a single one of the 

 ^ specimens of granite free from magnetic iron, although more was found 

 * in the younger ones than in those preceding them in age. The same 

 examinations carried on with a considerable number of trachorheites 

 from different localities give the invariable result that andesite con- 

 tains a large (comparatively speaking) percentage of this mineral; 

 trachyte less ; rhyolite still less ; the tuffs of these species giving results 

 in accordance with those shown by the rock to which they belong. The 

 rising percentage of silica as they are higher — j'ounger in the scale — 

 until rhyolite contains free silica; their approximate uniformity in 

 chemical composition throughout the world, which points to an origin 

 from similarly constituted material; and their almost unexceptional 

 occurrence in granitic countries, besides the great similarity of percent- 

 age which their constituents show, upon analysis, with the constitu- 

 ents of granite, seem to lead to the conclusion that they may owe their 

 origin to a remelting of granite. More careful and specific examina- 

 tions are required, however, to prove this hypothesis. With a view 

 to so doing, I have undertaken investigations into the questions in- 

 volved herein, and shall therefore do no more than suggest the idea 

 until results obtained justify a more complete and comprehensive as- 

 sertion. 



