ENDLicH.] ACIDIC VOLCANIC ERUPTIVES TRACHYTE. 225 



siou to study this series. So far as I am aware, do publication Las 

 been made with reference thereto since that time. The group is, hori- 

 zontally, a restricted one, but fraught with the occurrences of the high- 

 est interest. Not onl^' are the rocks themselves of very peculiar type 

 for the position they occupy, but the presence of many metalliferous 

 veins lends additional importance to the group. It occurs in the heart 

 of the Uncompahgre group and extends to its western edge. Some of 

 the highest mountains of that region are partly or wholly composed of 

 it. The latest discoveries of ore-bearing veins seem to have been made 

 at localities where this group occurs merely as capping the older ones. 

 With other words, instead of the veins being confined to No. 4, they 

 extend through it and can be reached in older formations. 



On the point of petrographic character this group presents more vari- 

 ations than any one of the volcanics. Many brilliant colors, of the 

 most delicate shades, are shown by the mountains and ridges. They 

 are due to admixtures of certain mineral substances. Dark colors may 

 be said to be characteristic of the main bulk of this group, but a very 

 prominent exception is made by what I have termed the '''•red stratum.^''* 

 Ori<iiually white, the presence of ferric oxygen compounds gradually 

 changes this color to yellow, orange, red, and brown. The rock is a 

 micro crystalline feldspathic paste of white color, containing very mi- 

 nute transparent crystals of sanidite and small crystals of pyrite. 

 Throughout the district, wherever this stratum could be traced, the 

 crystals of pyrite were contained in it as an "impregnation." Some of 

 the largest cubes seen will hardly measure 0.5 millimetre on the edge. 

 Decomposition of pyrite releases the sulphur and changes the iron from 

 a bisulphide to hydrated sesquioxide. This, in varying percentages, 

 produces the colors and shades above enumerated. 



Stating the general mineralogical character of the rocks of this 

 group, we may say that the paste is compact (rarely vesicular) and 

 micro-crystalline. Oligoclase is the most prominent of the enclosed 

 minerals. Another feldspar — tricliuic, probably andesite — occurs with 

 it. Sanidite may be regarded as an exception, and occurs only in the 

 lower members of the group. Pyrite, magnetite, hornblende, mica, and 

 chlorite are accessory minerals, of which the first-named occurs in the 

 .shape of an impregnation. 



. For trachyte No. 4 we can accept a thickness of 3,000 to 3,500 feet. 

 In its lower members it retains the stratoid character very well, but 

 higher up this is gr^^atly obliterated. From the fact that this series 

 forms the most rugged and typical mountains of the Uncompahgre 

 group, I term it the "Mountain group." 



Taking into consideration the general mineralogical constitution of 

 the rocks and the existence of ore-bearing lodes, we are tempted to paral- 

 lelize this group with propylite. This temptation has become all the 

 greater since the discovery of tellurides of gold and silver in the Hodg- 

 kiss Lode of Lake district. 



Kichthofen places propylite at the base of the volcanic series in our 

 Western Territories, and that position is accepted for it by all who have 

 acquaintance with the regions involved. It is merely for the purpose 

 of anticipating any comparisons that might be made, and to justify the 

 position 1 have assigned this group, that I enter into a brief discussion. 



It is well known that the lodes of Transylvania are found in a '■'■ green- 

 stone trachyte.''^ This rock has been referred to the " trachy tic series " 



*BulL U. S. Geol. Surv. No. 3, second series, 1875, p. 154 ; Eep. U. S. Geol. SuiV., 

 1874, p. 2^0. 



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