502 ERNEST HOWE 



north and east they are sharply bounded by fauh contacts with 

 sedimentary rocks of pre-Cambrian age, or have been intricately 

 infolded with them; another fault separates the greenstones on the 

 west from a large mass of granite, while to the southward they disap- 

 pear beneath the Paleozoic seclimentaries. 



Many of the exposures are of well-banded schistose rocks, and it 

 was only natural that they were at first assumed to be a part of the 

 great Archean complex of schists and gneisses known to occur near 

 by in the Animas Canyon. On closer study, however, it soon became 

 evident that there were marked differences between the two series of 

 rocks. The Irving formation, as the greenstone complex has been 

 named, from Irving Peak, was found to consist, not only of the schists 

 first observed, but also of massive basic rocks sometimes possessing a 

 porphyritic structure, others partly mashed or brecciated, and a few 

 distinctly granular, while no well-defined system of bedding or 

 stratification could be made out. All of the rocks are of a dull green- 

 ish color and appear to have undergone extensive alteration. At two 

 places massive quartzite was found, and at a number of localities 

 extremely sihceous schists occur, some of which have undoubtedly 

 been derived from quartzites through dynamic metamorphism, while 

 others must have been originally granites or closely allied rocks. A 

 single band of siliceous magnetite some fifteen feet in thickness was 

 also observed near the northern end of the series. 



Most of the exposures are a dull leaden-gray or green, in sharp 

 contrast to the lighter- colored granites and quartzites to the north 

 and west, and their somber tones add to the gloomy aspect of the 

 valley sides which have, in many places, been swept by destructive 

 forest fires. The dull monotony is occasionally relieved by dikes of 

 bright red granite porphyry or pegmatite near the contact with the 

 granite mass to the westward. The only marked variations in the 

 Irving formation itself are the comparatively rare occurrences of a 

 very Hght gray gneiss or mashed quartzite. 



Description oj the rocks. — The majority of the rocks found in the 

 Vallecito section display considerable textural variety, but appear 

 to be o nearly the same mineralogical composition. Hornblende, 

 chlorite, epidote, and rarely biotite can be recognized megascopically 

 in nearly all, and usually these dark minerals appear to be in excess 



