280 GEOLOGICAL SURVEY OF THE TEERITOBIES. 



severe tests. If ttie gaugue-rock is sufiaciently hard to preserve its out- 

 crop, or if the ores stain the soil, it becomes a comparatively easy task; 

 but when all traces of this kind are obliterated, he must resort to 

 uncovering the lode, unless, again by means of the appearance of the 

 vegetation, he can foUow it up. It has been mentioned above that 

 excavations, &c., serve to collect the percolating waters ; and although 

 the fissure may be filled, it wiU, as a rule, retain this quality to some 

 extent, so that, in a dry country, trees growing immediately upon the 

 lode will frequently show a more luxurious developmejit thin their 

 neighbors. 



For nearly horizontal veins and beds, boring is of importance, but 

 sometimes impracticable. 



Prospecting in our western countries is combined with so many hard- 

 ships and dangers that great credit is due to those men who spend 

 years of their lives in seeking for and developiog the mineral wealth of 

 their adopted home. 



PART I. 



In Gilpin County, the mining locality is found within a radius of about 

 three to four miles, starting from Central City, and is distinctly hmitable 

 at a glance by the perfect bareness of those hills in which the precious 

 metals are contained. Approaching Central from the east, through 

 Clear Creek Canon, a number of rounded hills are seen to the left — higher 

 ones to the right. Not a particle of timber remains on those that have 

 been prospected over; no low vegetation has had time to develop. 

 Black Hawk, at an elevation of 7,543 feet, Mountain and Central, of 

 8,300 feet, are seemingly one continuous town, although provided with 

 three city governments, located in Eureka Gulch, with mines on every 

 side, even in the very centers of the towns. Westward of Central, seve- 

 ral hills are located, also containing a number of wlodes, some of which 

 are being worked. On all sides, this comparatively small mining dis- 

 trict is surrounded by higher mountains, densely wooded, in which 

 every now and then an isolated lode is found. The geognostic features 

 in the immediate neighborhood of the mines are somewhat difficult to 

 determine with accuracy, owing to the great displacement of rocks that 

 has been occasioned by the search for lodes. Throughout the 'vicinity 

 of this mining locality, the main rock is a gneissic ©ne, showing numerous 

 changes and varieties, of which Mr. Marvine speaks ^more fully in his 

 report upon the geology of that section. In the immediate vicinity of the 

 mines, however, the rock is granitic, sometimes changing into that vari- 

 iety which has been termed aplyte, composed only of orthoclase and 

 quartz. Mostly, it is coarse-grained, inclined to separate in stratoid por- 

 tions, and yields readily to the decomposing action of atmospheric 

 influences. 



The accompanying geological map* (Plate I) will explain the dis- 

 tribution of the granite. Beginning at the summit of Quartz Hill, it 

 extends eastward, keeping a little to the north, with the connection 

 broken only at one point, until Eunning ^fiCill is reached. Composing 

 the main or entire portion of the small intervening hills, its limit south- 

 ward is defined by their own, on the north mainly by the gulch. This 

 granitic area is superficialiy entirely isolated fr-om any other, having a 

 main longitudinal direction of almost due east and west, a little north 

 of west. On Procer Hill, granite is again found, having the same strike, 



*T]ie drainage has been taken from the map published in Mr. Hague's mining report, 

 and also the roads, besides some of the lodes, the location of which corresponded with 

 mine. 



