EOTLicH.] GEOLOGY EELATIONS OF LODES. 295 



referring to Mr. Marvine's report for a more connected and comprehen- 

 sive discussion of these interesting features. 



Similar to the conditions observed at Central, the lodes here have 

 mainly two strikes, approximately east to west and northeast to south- 

 west, the former being the more numerous of the two. They show great 

 uniformity in dip, which is mostly near the vertical, and are rarely dis- 

 turbed. But few of the mines have reached any considerable depth, so 

 that not much can be said as yet about their character of persistency, 

 unless inferences might be drawn, judging from their formation, analo- 

 gous to that of other well-known localities. 



With reference to the relation that these lodes bear to the adjacent 

 rocks, it will scarcely be possible to apply a single rule covering all 

 cases. It may be said, I think, that the majority of them follow in 

 their course approximately the course of the porphyritic granite, are 

 sometimes contact between it and the granitic gneiss, sometimes have 

 their course within it, and in other cases stand in no connection with it. 

 This granite may probably be considered intrusive, although its age 

 will be by far prior to that of the Central City granite, possibly even 

 not much more recent than that of the surrounding rock. If the hy- 

 I)othesis of filling the fissures with their mineral-matter by the agent of 

 heat mainly were proved, the view expressed regarding the relative 

 age of these lodes would find some support, inasmuch as the metals 

 mainly found in them all have a lower temperature of volatilization than 

 those occurring chiefly at Central ; consequently would have been dis- 

 missed first from the common reservoir. Porphyry-dikes not infre- 

 quently occur, sometimes intersecting veins, but in no instance, so far 

 as could be observed, occasioning any disturbance. Usually the lodes 

 traverse the gneiss at some considerable angle to its dip ; only in rare 

 cases do they strike and incline parallel with it. In the latter case they 

 seem to be younger than in the former, judging from their mineralogi- 

 cal and geognostic character. 



Dikes of hornblende-rock, with epidote, resembling diorite, are quite 

 frequent in the gneiss, and are almost invariably accompanied by a nar- 

 row seam of minerals, mostly galena and blende. Their strike is at 

 right angles to that of the ore- veins, as at Central, running from 

 north to south. 



Due west of Georgetown, 8,412 feet above the sea, Leavenworth 

 Mountain rises up about 1,200 feet above the valley, and it is on 

 the south side of this mountain that a number of very valuable lodes 

 were found. The Colorado Central, having a course about east 10° 

 north, is located on the south sid6 of this mountain, about 700 feet 

 above the creek. It is a very rich silver-mine as far as the character 

 of its ore is concerned, and has paid well ever since it was worked. 

 Between walls the vein is very wide, no wall-rock having been found 

 on the south side as yet, although nearly 30 feet of vein-matter have 

 been cut through. The north wall is granite and well defined ; dip 

 slightly to the north. Ore traverses the entire gangue-rock, mainly in 

 the direction of east to west, branching off, however, at several points 

 into seams of one-fourth inch to an inch in thickness, which, neverthe- 

 less,- pay following, owing to its rich character, but these spurs consoli- 

 date at places, and form a solid vein of ore 1^ feet thick. Near the surface 

 of the lode, a mass of float-ore occurs, of the same mineralogical charac- 

 ter as that found at lower depths, although its position there seems 

 somewhat out of place if it is from the same vein. Quartz and feld- 

 spar compose the gangue-rock — the latter mostly decomposed— both 



