144 GEOLOGICAL SUEVEY OF TPIE TEREITORIES. 



Luis division of the survey. A complete knowledge of the general 

 geology of the region as a whole, as well as of these ore-deposits, would 

 probably show connections between the two, and lead, as in other 

 regions, to results valuable to the mining-engineer. To trace such con- 

 nections, and to master the relations of the veins and their contents to 

 the formation as a whole, could not be expected to be accomplished in a 

 single season's work over an extended area. 



A number of porphyry dikes, usually of short extent, have penetrated 

 the metamorphic rocks at many points, perhaps most noticeable in the 

 region drained by Left-Hand, Four-Mile, and adjacent creeks. They 

 have considerable variation of composition, but have not yet been made 

 the subject of special examination by the survey. It is on either side 

 of one of these porphyry dikes, along the planes of contact between it 

 and the inclosing granite, that the rare and interesting telluride ores 

 of Gold Hill are mined. 



Dr. Endlich's analysis of specimens from this locality has revealed 

 some new mineral species, while specimens of this and neighboring 

 porphyries furnished with notes upon their occurrence t(f Professor -Ben- 

 jamin Silliman, jr., of New Haven, have formed the subject of an exami- 

 nation by him, in the American Journal of Science and Arts for July. 

 The description of these ores, and their mode of occurrence, is given in 

 the chapter on mines in Dr. Endlich's report. 



THE STRATIGRAPHY OF THE EAST SLOPE OF THE FRONT RANGE. 



Three causes combine to render the rapid study of the stratigraphy of 

 the archajan rocks difficult and its results uncertain : First, their struc- 

 ture is not only often complex, but obscure, the evidence of it being at 

 times nearly or wholly obliterated by the metamorphism, and often over 

 large areas very difficult to find; second, this metamorphism renders 

 lithological characters inconstant, so that a stratum that at one point 

 may be characteristic among its neighbors, may, at another, become 

 like them, or all may change so as to retain none of their geological fea- 

 tures, becoming again like other series, so that lithological resemblances 

 cannot often be taken as a guide to follow, and may even become mis- 

 leading; third, the erosion producing the present surface features ol 

 the mountain region had the direction of its action determined by 

 movements of the surface which were not closely connected with the 

 extended plications of its rocks ; and, moreover, since this erosion has 

 not long been active among these rocks, there appears no well-defined 

 connection between the topography and the structural geology. The 

 ancient erosion gradually wore down the mass to the surface of the 

 sea, and while previously to this it was no doubt directed by the struc- 

 ture, yet the mass was finally leveled off irrespective of structure or 

 relative hardnesses of its beds by the encroaching ocean, which worked 

 over its ruins and laid them down upon the smoothed surface in the 

 form of the Triassic and other beds. The recent great uplift, while it 

 probably added new plications to the accumulated plications of the past 

 in the ancient rocks, was quite simple with respect to their total plica- 

 tion, and left the upper Triassic and other sedimentary beds compara- 

 tively simply structured, they having been affected alone by the later 

 movements. 



As the mass appeared above the sea and surface erosion once more com- 

 menced, but which now acts upon the recent rocks covering probably in 

 greater part the complex underlying rocks, it was directed off from the line 

 of greater uplift down the long slopesof the rising continent to the retiring 



