312 GEOLOGY AND MINING INDUSTRY OF LEADVILLE 



Cottonwood granite, as far as can be seen, formed only a single massive 

 body without ramifications. The porphyries must therefore have had more 

 superfluous heat than the granite to devote to the work of melting np the 

 included masses of sedimentary rocks, and one can see here, as one cannot 

 in the granite, that such masses were actually caught up and included in 

 the fused rock. It would be fair to assume, therefore, that in this case rela- 

 tively larger amounts of sedimentary rocks would have been fused and that 

 the evidence of such fusion would be more apparent. 



In the present condition of microscopical investigation we may trace the 

 development of one mineral from another and detect its most minute altera- 

 tion, either by fusion or by chemical interchange; and, had any of these sedi- 

 mentary rocks been assimilated into the igneous mass, it would seem hardly 

 possible that every trace of the process should have escaped our observa- 

 tion in the thousands of rock sections that have been examined. In point 

 of fact, however, although in the case of the porphyrite dikes the eruptive 

 material is found to fill minute cracks in the inclosed fragments of Archean 

 rocks, there could be detected no evidence of fusion on either adjoining or 

 inclosed sedimentary rocks. In the eruptive rocks themselves, moreover, the 

 alterations of mineral constituents are all the result of secondary processes 

 after the mass had fully cooled and crystallized. 



The testimony of the chemical composition of these rocks is, so far as 

 it goes, equally opposed to the supposition that foreign matter has been 

 assimilated by any of these intrusive bodies. Of White Porphyry too few 

 specimens were analyzed to afford a decisive test; but it is to be remarked 

 that the two specimens (see Table II, Appendix B) which show an abnor- 

 mally high percentage of silica are from the London and New York mines 

 and are extremely decomposed and altered, a secondary action which has 

 decreased the proportion of more soluble basic constituents and correspond- 

 ingly increased the percentage of silica. Of the Lincoln or Gray Porphyry 

 six specimens from different bodies show an average of 68.08 per cent, of 

 silica, with an extreme variation from this average of 2.63. For the com- 

 bined alkalies, three specimens show an average of 6.14 and an extreme 

 variation of 0.86; and for lime and magnesia combined, three specimens 

 show an average of 4.03, with an extreme variation of 0.19. 



