184 G. H. Stone — Granitic Breccias of Colorado. 



Art. XYIII. — The Granitic Breccias of Grizzly Peak, 

 Colorado ; by George H. Stone. 



Grizzly Peak is situated about twenty- three miles southwest 

 of Leadville, and forms the watershed from which streams 

 flow northwest into the Roaring Fork and the Grand, south- 

 west into the Gunnison, and east into the Arkansas. Its ele- 

 vation is marked by Hayden as 13,956 feet. It is the highest 

 pinnacle of a very irregular volcanic mass consisting of several 

 radiating ridges one to five miles long. 



The eruptive rocks of the district are marked on Hay den's 

 Atlas of Colorado as rhyolite. They contain a large amount 

 of uncombined silica, but sanidine is not abundant, and they 

 more nearly resemble the quartz porphyries of Leadville than 

 the ordinary sanidine-bearing rhyolites of central Colorado. 

 Dark silicates occur only in small quantities locally. The 

 volcanic mass is situated in the midst of a region of schists 

 containing dikes and irregular bodies of granite. 



The volcanic ridges have steep lateral slopes. Cliffs abound, 

 though in many places the rock disintegrates so as to form a 

 smooth talus of small fragments. There are a few somewhat 

 symmetrical cones, but the general outline of the ridges is 

 rugged, and their tops form a sierra. In several places large 

 areas are colored bright red, and the local name for the central 

 mass where Grizzly Peak is situated, is Red Mountain. The 

 color is due more to the oxidation of pyrite than to the decom- 

 position of iron-bearing silicates. 



Wherever the lava is exposed on the surface, as it often is 

 toward the tops of the ridges and on some projecting bosses 

 and crags, it is somewhat scoriaceous and decomposed. But 

 over most of the higher portions of the ridges the lava is 

 capped by a layer of breccia composed mostly of fragments of 

 the lava that are only a little polished and rounded at the angles. 

 In some places are many subangular fragments of granite or 

 schists lying upon or among the lava fragments, often being 

 so abundant as to form all of the surface layer of the breccia 

 near the top of the mountains. I saw a granite bowlder six 

 feet in diameter embedded in a solid breccia consisting of lava 

 fragments. 



In some cases the volcanic rock extends to near the bottoms 

 of ^ the ridges, but in general it forms the upper half or two- 

 thirds of the ridges. Over the lower portions of the volcanic 

 ridges and thence extending for a short distance down the slopes 

 over the adjacent granite and schists, we find a superficial 

 layer of firmly cemented breccia consisting wholly of rather 



