276 UNIVERSITY OF COLORADO STUDIES 



have been described by every name from andesite to phonolite in different reports. They 

 do vary from the composition of andesite to that of trachyte, but in the area specially 

 studied they are of intermediate composition, and may be called Latite Porphyry. 



It is very possible that they are the necks of surface flows vv'hich furnished the andesite 

 pebbles for the beds of the Denver Formation. 



Microscopic. — Under the microscope the porphyries vary from a rock composed of 

 a holocrystalline ground-mass of quartz and feldspar with no phenocrysts, to a rock with 

 little ground-mass and resembhng the true granite. As a rule, the ground-mass is a fine- 

 grained granitic mosaic, and might be called a micro-granite. In many specimens, 

 especially those that are highly iron-stained and rich in magnetite and hematite, the feld- 

 spars are lath-shaped, and the quartz is allotriomorphic to the feldspar and arranged in 

 the spaces left over after the consohdation of the feldspar. Here in many cases the feld- 

 spars are iron-stained all through; in other cases the stain has not penetrated to the center. 

 The lath-shaped feldspars have here usually taken on a rough flow structure. In the case 

 of vein breccias and the more andesitic varieties the ground-mass may resemble a felted 

 paste. The highest powers of the microscope fail to resolve this paste into its component 

 minerals. 



In the ground-mass, of whatever character, as a rule are larger crystals of phenocrysts 

 of one or more generation. The larger crystals are generally plagioclase, with some 

 orthoclase. These nearly always show evidences of zonal development, and are of intra- 

 telluric formation. The feldspars frequently show evidence of rebuilding. The crystals 

 of a second growth are the same as those of the first growth, but, as a rule, rather more 

 acid, with quartz, biotite, and hornblende. In no case in this area has the quartz been 

 seen in large crystals, though one dike on Mill Creek studied outside of the area shows 

 good quartz phenocrysts, with perfect crystalline outlines. All these larger crystals have 

 suffered more or less from corrosion by the magma before the consolidation of the ground- 

 mass. This is especially true of the biotite and hornblende crystals, of which in many 

 cases only the skeletons remain for identification. In one case portions of the ground- 

 mass were noticed entirely inclosed in a phenocryst of feldspar. Among the earliest con- 

 solidations are apatite and titanite, frequently in relatively large and well-formed crystals. 

 In the ground-mass, and in addition to the quartz and feldspar previously noticed, are 

 found pyrite, magnetite, biotite, hornblende, augite, hematite, zircon, and in one case 

 fluorite. Sericite, epidote, zoisite, calcite are present as alteration products, with frequent 

 secondary quartz. 



Pleistocene and Recent 



At several places noted previously under glaciation, and also at various points in 

 Clear Creek Canon, gravel deposits have accumulated. On the northwest portion of the 

 Evergreen Quadrangle by the Silver Age Mill on the north side of Clear Creek, and also 

 nearly opposite Sawmill Gulch, on the north side of Clear Creek, the remains of an old 

 stream terrace can be seen about one hundred feet above the present stream-bed. These 

 correspond to the old terraces which are more prominently developed on the Georgetown 

 Quadrangle around Idaho Springs. Some of these are more or less completely consoli- 

 dated into conglomerates. 



Sediments 



No attempt has been made to map the sediments which have been described in the 

 Denver Monograph. 



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