310 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



in the changing of sandstone to quartzite and of limestones to marble, but 

 these are by no means contact phenomena, and occur as often, if not oft- 

 ener, at considerable distances from the granite as in direct contact with it. 

 Porphyry dikes also cross the sedimentary strata in the vicinity, but these 

 have no more necessary connection with the granite than have the neigh- 

 boring bodies of volcanic rocks. They are not direct offshoots from it, and, 

 so far as their manner of occurrence and structure go, may bear the same 

 relation to it that the porphyries of the Mosquito Range do to the Archean 

 eruptive granite. When I examined this region on the Exploration of the 

 Fortieth Parallel, my first impulse, guided by my teachings as a student of 

 geology, was to consider the granite an intrusive mass cutting Carboniferous 

 strata ; it was, however, difficult to conceive that it should have eaten up 

 over five hundred cubic miles of sedimentary rocks without leaving some 

 more definite evidence of this action than it has. This, together with other 

 considerations, led me, after a careful weighing of the evidence, to the view 

 that the granite must have been erupted in Archean time, and that in the 

 ocean of the Cambrian and subsequent periods it formed a submerged reef 

 around which the sedimentary beds were deposited. Professor A. Geikie, the 

 English geologist, whose eminent ability none can recognize more full}" and 

 heartily than I do, after a visit to the region, occupying only a few days, 

 decided promptly that my view was wrong, and, evidently basing his opinion 

 on the granite bosses of his own country, has published it in his text-book 1 

 as an instance of Post-Carboniferous granite. While, owing to the necessa- 

 rily hasty character of reconnaissance work like that of the fortieth parallel, 

 it is very possible that a more detailed study might lead us to modify our own 

 views, especially in regard to so complicated a district as that in question, 

 I should still be unwilling to admit, even at the instance of so experienced 

 a geologist as Professor Geikie, that the Cottonwood granite can be Post 

 Carboniferous, even if my only reason were that I do not admit the possi- 

 bility that the granite had eaten up or assimilated this enormous mass of 

 sedimentary rocks without leaving any trace of fusion on the adjoining 

 rocks, any incompletely assimilated portions within its own mass, or with- 

 out showing in its own structure and composition any marked variation from 

 that of the normal rock. 



'Op. cit., p. 646. 



