282 GEOLOGY AND MINING INDUSTEY OF LEADVILLE. 



geneous-looking rock, resembling yellow beeswax, which proves by analysis 

 to be an intimate mixture of calcite and serpentine. A complete analysis 

 of the soft green material from a specimen of the darker-colored rock is given 

 in Analysis I, Table VII, Appendix B, which proves it to be an almost normal 

 serpentine, the oxygen ratio being 3 : 3.95 : 2.11, instead of 3:4:2, which is 

 the theoretical proportion. Analysis II, in the same table, is that of the whole 

 mass of yellow rock, which is found to contain 57.57 per cent, of carbonate 

 of lime. If this be deducted, the composition of the residue is essentially 

 the same as that of I. The microscope confirms the conclusion that the 

 rock is a simple mixture of serpentine and calcite, as no other mineral can 

 be distinguished by it. It also shows that the major part of the rock is 

 in grains which show the cleavage distinctly, whereas the small grains of 

 calcite which are sometimes found in the dolomites show no such cleavage 

 lines ; hence it is evident that the calcite has been recrystallized. 



Origin of the serpentine. — It, is evideut, froui the manner of its occurrence 

 in and intimate admixture with the sedimentary rocks, that the serpentine 

 is not of eruptive origin. It seems equally improbable, from its extremely 

 local development, that it could have been formed at the time of the pre- 

 cipitation and deposition of the original sediments. It is noteworthy, 

 further, that the localities where it was found have been near centers of 

 eruptive action and of consequent intense metamorphism. The dolomites 

 of this horizon, which are all more or less silicious, contain all the constit- 

 uent elements of serpentine, except water. If by the addition of this ele- 

 ment a reaction between it and the silica and magnesia could be brought 

 about, serpentine might have been formed directly from the dolomites. As, 

 however, it is difficult to conceive of such a direct reaction, it seems better 

 to seek some intermediate step. Among the specimens of serpentinous rock 

 from the Red am])hitheater, one has gra3' portions, comparatively free from 

 serpentine, in which fibrous silky crystals can be observed. The nncroscope 

 showed that these are amphibole crystals, and, further, that pj-roxene was 

 present. Analysis III, Table VII, shows the conqDOsition of these silky 

 crystals after thej* had been separated from the rest of the mass, whicli is 

 ])racticall}' that of actinolite. The part supposed to be pj-roxene, wliich is 

 distin-o'uishable as being less lustrous, was not analyzed. Now, the for- 



