86 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



Occurrence. The manner of occurrence of this rock in the region above 

 mentioned is quite remarkable. It has been traced in practical continuity 

 over an area of some four square miles, and probably has a much wider 

 extent. It is regularly interbedded and rarely over twenty feet in thick- 

 ness. It is easily traceable from a distance on the cliff walls, as a dark 

 band between the lighter-colored sedimentary strata, and, while it appar- 

 ently follows rigorously the same horizon, it is found, on close examina- 

 tion, to cross from bed to bed at different points, so that its range in this 

 area is actually from the upper part of the Cambrian to the top of the Silu- 

 rian. The manner in which it crosses the beds is shown in Plates XIII 

 and XIV. It also occurs at various other points in narrow dikes in the 

 Archean. 



This rock forms Type V of Division 13 of Mr. Cross's classification, this 

 division being that in which the hornblende and biotite are found both in 

 the groundmass and as porphyritic constituents. His Division A includes 

 rocks in which these basic minerals are entirely wanting in the groundmass, 

 and which, in consequence, are of much lighter color than either of the 

 other divisions. The rocks of his Division C, on the other hand, in which 

 the hornblende and biotite are found only in the groundmass, are generally 

 of darker color, and the arrangement of these minerals around the larger 

 porphyritic crystals often shows a fluidal structure. 



Included fragments of pebbles of Archean rocks are more frequent in 

 these than in any other eruptive rocks of the region, and in Plate VII, Fig. 1, 

 is shown a specimen of a rock of Division A, from a remarkable dike in the 

 Arkansas Amphitheater, in which the included fragments are large rounded 

 crystals of orthoclase, whose presence in such form it has not yet been pos- 

 sible to account for. 



TERTIARY ERUPTIVES. 



The Tertiary eruptives found in this region consist of rhyolites and one 

 occurrence of quartziferous trachyte within the limits of the Mosquito map, 

 and of an interesting occurrence of andesite just south of those limits. The 

 quartziferous trachyte being a small body, and of no great importance a 

 bearing on the subject-matter of this report, has not been designated by a 

 special color, but is included on the map under the rhyolite color. The 



