GOLD HILL MINING-REGION: ITS POSITION AND GENERAL 



GEOLOGY. 



By Arch. E. Marvixe.* 



The accompanying map has been prepared to show the general out- 

 lines of the country in the neighborhood of the Gold Hill and Ward 

 miniugdistricts, in the mountains of Colorado, while the following 

 remarks explain briefly the position of the region in relatiou to the 

 surrounding country, as well as its general geology, as determined dur- 

 ing the explorations of the summer of 1873. 



Nearly parallel with, and a few miles west of, the western border of 

 the country represented on the map, rises the main continental "divide" 

 in a north-and-south crest, which here reaches an altitude of nearly 

 13,000 feet above the sea-level. From the base of the main crest, a zone 

 of mountainous country extends eastward, which is cut through by the 

 streams in a general east-and-west direction. 



The intervening ridges are not sharp, but of a massive character, 

 often with undulating surfaces, their higher points usually reaching in 

 general a pretty uniform level, the ruggedness of the country being 

 produced by the deep canons of the stream. It is a portion of this 

 region that is represented on the map. 



At the east (near the border of the map) the region abruptly ends 

 along a nearly north-and-south line, the massive si)urs falling to the 

 zone of "hog-backs,-' or ridges of upturned sedimentary rocks, which 

 lie all along the base of the range. 



The "red beds," probably of Triassic age, form the innermost ridge, 

 lying directly on the Archaean rocks of the mountains. These, in going 

 eastward, are followed by the upturned edges of Jurassic shales, the 

 Cretaceous groups, and the great Lignitic formation, of as yet disputed 

 Cretaceous or Eocene age, which stretches eastward, and forms the beds 

 directly underlying the great plains. 



Boulder City is on the border between the mountains and plains, and 

 is reached by railroad, Denver City being but twenty-five miles to the 

 south and east. From Boulder City, wagon-roads up the various caiions 

 give access to the mines in the mountains. South of the region repre- 

 sented on the map, from fifteen to twenty miles, is Clear Creek, much 

 like the Boulder in general character, on the tributaries of which the 

 better known mining-regions of Georgetown, Idaho, Empire, Black 

 Hawk, and Central City are situated. 



The rocks of the mountains, as a whole, may be considered as being 

 comi^osed of a great series of metamorphic rocks of Presilurian age. 

 Quartzites, siliceous, micaceous, some hornblendic and garnetiferous 



*Tbe following notes were originally prepared to accompany an article of Prf)f. 

 Benjamin Silllman,.ir., in the July number of the American Journal of Science, on the 

 telluride ores of Colorado. 



Tliey should have been printed with Mr. Jlarvine's report, but were accidentally 

 omitted ; but as they refer to the area occupied by the survey, and should form y. por- 

 tion of the report proper, it seems advisable to print them in this conuectiou. 



