GEOLOGY. 241 



other fossils which characterize that horizon. Associated with 

 these were found several new species of mammals and birds. 



"This interesting series of fresh-water tertiary strata lies 

 almost horizontal, dipping apparently, but very slightly, towards 

 the north-east. It probably forms the south-west border of the 

 great Miocene Lake Basin east of the Rocky Mountains, which 

 is so remarkable for its extinct animal remains. Our party traced 

 the same formation with its more common fossils about 30 miles 

 north-east into Wyoming, along the hills known as Chalk Bluffs, 

 and still farther north in the Pine Bluff range." Amer. Jour. 

 Science, September, 1870. 



GOLD AND SILVER MINES OF COLORADO. 



The gold and silver lodes of this territory, so far as they are 

 observed, are entirely composed of the gneissic and granitic rocks, 

 possibly of the age of the Laurentian series of Canada. At any 

 rate, all the gold-bearing rocks about Central City are most 

 distinctly gneissic, while those containing silver, at Georgetown, 

 are both gneissic and granitic. The mountains in which the 

 Baker, Brown, Coin, Terrible, and some other rich lodes are 

 located is composed mostly of gneissic and reddish feldspathic 

 granite, while the LeavenworthandMcClellan Mountains, equally 

 rich in silver, are composed of banded gneiss with the lines of 

 bedding or stratification very distinct. 



There is an important question that suggests itself to one 

 attempting to study the mines of Colorado, and that is the cause 

 of the wonderful parallelism of the lodes, the greater portion of 

 them taking one general direction or strike, north-east and 

 south-west. We must at once regard the cause as deep-seated 

 and general, for we find most of the veins or lodes are true 

 fissures, and do not diminish in richness as they are sunk deeper 

 into the earth. All these lodes have more or less clearly defined 

 walls, and some of them are quite remarkable for their smooth- 

 ness and regularity. We assume the position that the filling up 

 of all these lodes or veins with mineral matter was an event sub- 

 sequent to any change that may have occurred in the country 

 rock. Now, if we look carefully at all the azoic rock in this 

 region, we shall find more or less distinctly defined, depending 

 upon the structure of the rock itself, two planes of clearage, 

 one of them with a strike north-east and south-west, and the 

 other south-east and north-west. Besides these two sets of 

 cleavage planes, there are in most cases distinct lines of bedding. 

 The question arises, What relation do these veins hold to these 

 lines of cleavage ? Is it not possible that they occupy these 

 cleavage openings in lines of greatest weakness? 



I have taken the direction of these two sets of cleavage planes 

 many times with a compass, over a large area, and very seldom 

 do they diverge to any great extent from these two directions, 

 north-east and south-west or south-east and north-west. In some 

 instances the north-west and south-east plane would flex around 

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