GEOLOGICAL SUEVEY OF THE TERRITORIES. 187 



CHAPTER XI. 



THE GOLD AND SILVER MINES OF COLORADO. 



I will confine my remarks mostly to the geological features connected 

 with, these mines, inasmuch as Mr. Frazer, in his report appended, has 

 fully treated this subject. 



The gold- and silver lodes of this Territory, so far as they are observed, 

 are entirely composed of the gneissic and granite rocks, possibly rocks 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 feld- 

 spathic granite, while the Leavenworth and McClellan Mountains, 

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

 bedding or stratification very distinct. 



There is an important c^uestion 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, northeast and southwest. We must at once regard 

 the cause as deep-seated and general, for we find that 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 subsequent to 

 any change that may have occurred in the country rock. Now, if we 

 look carefully at all the azoic rocks in this region we shall find more or 

 less distinctly defined, depending upon the structure of the rock itself, 

 two planes of cleavage, one of them with a strike northeast and south- 

 west, and the other southeast and northwest. Beside 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 x)ossible that they occupy these cleavage openings 

 as lines of greatest weakness ? 



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

 times with a comjDass, over a large area, and very seldom do they diverge 

 to any great extent from these two directions, northeast and southwest 

 or southeast and northwest. In some instances the northwest and south- 

 east plane would flex around so as to strike north and south, and the other 

 one so as to trend east and west, but this is quite seldom, and never occurs 

 unless there has been some marked disturbance of the rocks. There are, 

 however, a few lodes which are called " east and west lodes," and some, 

 " north and south." A few have a strike northwest and southeast, but 

 are generally very narrow and break off from the northeast and south- 

 west lodes, are very rich for a time and then "pinch" out. It would 

 seem therefore quite possible that the northeast and southwest veins 

 took the lines of cleavage in that direction as lines of greatest weakness, 

 and that the northwest and southeast lines cross the other set, ami 

 that a portion of the mineral material might accumulate in that cleavage 

 fissure. I merely throw out this as a hint at this time, which I wish to fol- 

 low out in my future studies. I am inclined to believe that the problem of 

 the history of the Rocky Mountain ranges is closely connected with 

 these two great sets of cleavage lines. As I have before stated, my own 

 observations point to the conclusion that the general strike of the met- 



