338 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



scopic elements, standing in marked contrast to tliose forms under Division A. Between 

 these extremes, in regard to botii structure and composition, are the forms embraced 

 under B. In these the groundmass contains more or less of one or both of the dark 

 basic minerals, and in proportion as these minerals enter into the composition of the 

 groundmass the macroscopic elements become less distinct, thus forming a gradual 

 transition to the Division C. 



Division A The plagioclase usually stands out very plainly in these rocks, and it 



is evident that no orthoclase is present in macroscopic individuals. Quartz occurs in 

 good crystals and rather plentifully. The groundmass is microcrystalline and pos- 

 sesses a very regular granular structure, its components being almost exclusively quartz 

 and orthoclase. A dike in gneiss, near a little lake northwest of Mount Lincoln, rep- 

 resents the typical hornblendic variety [120], while a similar dike in North Mosquito 

 amphitheater is of the corresponding biotite rock [119]. Several occurrences at the 

 head of Buckskin gulch are nearly allied to these type rocks. 



Division B. — By far the larger number of the i)orphyrites in the series fall within 

 this division. In the three subdivisions of the table, one or both of the heavier sili- 

 cates appear in the groundnmss as well as in larger crystals. If the groundmass 

 minerals are regarded as belonging to a second phase of the rock's existence, one 

 of the striking peculiarities of this division is most natural, while from another point 

 of view it might seem strange. The peculiarity referred to is the observed indepen- 

 dence of the daik basic silicates occurring in the groundmass, of the species which 

 may be developed as macroscopic constituents. The formation of hornblende in 

 numerous large crystals during the first period of consolidation does not necessarily 

 demand that the same mineral should be developed in the second period. The changed 

 conditions attending the final consolidation may produce biotite or hornblende, or both 

 of them, uninfluenced, or at least uncontrolled, by the earlier crystallizations. The 

 table above shows this, but a study of the variations in the different rocks collected 

 makes tlie fact much plainer. The rock most frequently met with in all the district 

 belongs to Type V of this division. It is the one found in the intrusive sheets on the 

 sides of Mosquito [127] and Buckskin gulches [120], on Mount Lincoln, or in dikes in 

 the Archean, as on Bartlett Mountain [124] and Democrat Mountain [259]. The lower 

 figure of Plate VII, page 84, shows the macroscopical appearance of this rock very 

 ■well. Hornblende crystals are frequently well terminated in this modification, and 

 owing to the minute size of many well-shaped prisms, while all intermediate stages 

 are also represented, it becomes diificult to decide whether there has or has not been a re- 

 currence in the formation of hornblende ]irisms with good crystal form. Fig. 3, Plate 

 XX, was designed to show both large and small prisms of hornblende with good ter- 

 minal planes, but the imperfect execution of the prints leaves much to the imagination. 

 In Fig. 4 of the same plate are shown needles of hornblende with the more common, 

 irregular terminations. 



Division c._The compact rocks of this division are not very numerous. The two 

 occurrences illustrating best the micaceous and hornblendic varieties occur together 

 'in North :Mosquito amphitheater. One of these, the biotite rock [260], was analyzed 

 (p. 340), and its micro-structure is indicated by Fig. 1, Plate XX. Two other compact 

 rocks deserve special mention under the next heading. 



