* 
26 B. Sillinan—Mineralogical Notes. 
It appears from them, in general, that near the mining hamlet 
of Gold Hill, about twenty- -five miles northwest of Denver City, 
and at an elevation of almost 8,000 feet above tide, is a wide 
dike of prophyry cutting the metamorphic rocks, probably of 
Archean age, about six miles west of where the Triassic rocks 
die out at the base of the mountains. A ss of this dike, A 
furnished by Mr. Marvine, 
is annexed, showing the tel- 
lurium- -bearing veins B and 
con its sides. The porphy- 
ry of which it is composed 
has distinct crystals of feld- 
Se oe ed - spar implanted in a_pur- 
a. Porphyry dyke. 8, as Vein with gold — gray paste. These 
and tellurium stals- have a greenish- 
white color, and are mas oan Nocerncoatl, As seen in 
a microscopic section, it shows the usual obscurely crystalline 
ground-mass of felsite, with crystals of quartz, and sections of 
feldspar crystals showing the parallel bands of a triclinic species. 
A glance at the map shows the position and course of this dike, 
ee also the existence of ren dikes of porphyry in the same 
. The porphyry from “7-30” and “Central” Mines 
honed sciivendtie that from “Red Cloud,” while that from a 
dike (No. 136) between the “7:30” and the “ Americus” is dis- 
tinctly SS and that from the “‘ Niwot” Mine, at the west 
margin of the map (No. 181), is a quartz- porphyry, with dis- 
tinct crystals of biaxial mica. Those from the dikes at Jim 
Town (specimens No. cide on the north border of the district, 
are distinct sanadin-trachyte. 
The tellurium ores have been explored, so far, only in con- 
nection with the dike near Gold Hill, shown in the section, 
although they exist with the dike at wy 30” and the “Central.” 
These ores are found along the line of contact of the walls of 
the dike, in a quartz g: ae associated chiefly with pyrite in 
small, brilliant, highly - ified crystals, and rarely with chalco- 
pe and sphalerite. Prof. Hill speaks (loc. cit.) of lead; but 
I have a no salts re this etal in the specimens receiv 
quartz is chiefly hornstone and uncrystalline quartz, and, on 
the side of the country rock, it is mixed with feldspar. Native 
gold is not visible in any of the specimens IJ have seen of this 
ore from below the surface ; but where the surface is weathered, 
it exhibits free gold, arising from the decomposition of the 
tellurets. 
On the sides of the dike the line of division is clearly 
defined, but not so on the side adjacent to the metamo “4 
rocks, it blending on this side with the granitic materials. 
thickness of the veins varies from four or six feet toa few 
