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near station 10, a subsidence that covered a considerable area of ground. 

 From various sides the dip of strata can be noticed to incline toward the 

 region a little north of the station, while outside they dip off again. In 

 the Annual Eeport of the United States Geological Survey, these facts 

 will be more thoroughly elucidated, and treated of at length. 



Toward the southwest some suiall outbursts of volcanic material ap- 

 pear to have taken place, but their extent was limited, their flows of 

 small dimensions only. Local phenomena near station 10 lead to the 

 conclusion, that after the eruption was passed, a subsidence took place 

 near that point, amounting to several thousand feet. We find on high 

 peaks, sometimes more than twenty and thirty miles distant, caps of ba- 

 salt and dolerite, as well as rhyolite, that are either horizontal or very 

 nearly so. This points, even without their lithological affinities, to a 

 continuous flow of lava over a large area of country, at a time when the 

 elevation was almost, if not entirely, as high as at the present day. 



My purpose at present is to speak more particularly of the mining 

 region of the San Juan district ; I shall therefore abbreviate these more 

 general remarks upon the geology of the region as much as possible. 

 Certain typical points were found, where the rocks of the several strata 

 were particularly well developed, and it may be of use to give a synop- 

 sis of their mineralogical constitution. 



The rocks of No. 1, as found on White Earth Creek, and a number of 

 other localities, are very readily recognized by the peculiar texture and 

 structure. In color they usually vary from a white to light yellow, 

 gray, greenish into green, red, brown, and sometimes almost black. On 

 the Eio Grande, near Lost Trail Creek, the development and exposure 

 of this stratum are particularly characteristic. Weathering in columnar 

 masses, their general appearance is not unlike many of the " monu- 

 ments" of the Pike's Peak region. Many colored strata succeed each 

 other, nearly horizontally deposited. Mainly feldspathic material, de- 

 composing on the surface, composes the rock, while the colors are pro- 

 duced by oxidized compounds of iron. Small elongated crystals, of a 

 dark-green hornblende, and colorless crystals of sanidite are dis- 

 persed throughout the paste. Some strata, more particularly the upper 

 ones, show a fine-grained ashy color and texture, while others present 

 the appearance of having been redeposited by water. No. 2 shows 

 colors in which the lilac and light brown-tints are predominating. The 

 paste is compact, sometimes slightly vesicular, with numerous small 

 crystals of sanidite dispersed throughout. Small six-sided prisms of 

 black mica are associated with it, as well as crystals of a splendent 

 brown mica. Along the Eio Grande, below Lost Trail and Pole Creeks, 

 this formation is particularly well developed, and its strata can often be 

 traced for miles. At some localities the sanidite is adularizing. In the 

 upper portion of No. 2 bands or nodules of a porphyritic pitchstone are 

 found, imparting to the bluffs a "tigered" appearance. Black mica 

 and sanidite are the segregated minerals. Between stations 21 and 22 

 a stratum of this peculiar rock can be seen exposed for a distance of 

 about three miles, having a thickness of two to five feet. These occur- 

 rences are characteristic for the upper No. 2 and lower No. 3. 



The colors of No. 3 are usually darker than those of the preceding 

 group, and it can readily be divided into two divisions. No. 3 lower 

 shows a laminated texture, the laminae being horizontal in their original 

 position. A great many sanidite crystals are found in it, and but few 

 of mica. Paste is somewhat vesicular, more so on the surface than on 

 a fresh break, as there the vesicles are formed by the decomposition of 

 sanidite. No. 3 upper contains less sanidite, more mica, and gen- 



