304 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



the difference of internal structure in a given species, due to difference 

 of pressure, is, if it exists at all, so slight as to escape observation. Thus, 

 between the lowest body of White Porphyry, which occurs in the Archean, 

 and the highest, which is near the top of the Weber Grits (a vertical range 

 of about three thousand feet), no essential difference in internal structure was 

 detected. It would appear, therefore, that, while very wide differences in 

 the conditions of cooling may produce a generic difference between two 

 series of rock varieties, the internal structure of a given variety is not 

 dependent on those conditions alone, but that the species possesses certain 

 essential characteristics of its own which are dependent on other foctors. 



While the petrographical studies made in the course of this investiga- 

 tion, forming only an accessor}' and not an essential part of it and being 

 confined to a limited area, are not sufficiently complete to form the basis of 

 an essential change in the classifications hitherto adopted, they point decid- 

 edly to the fast approaching necessity of some essential modification in 

 them. Thus, the White and Lincoln Porphyries would a few years ago 

 have been unhesitatingly classed by petrographers, from a study of their 

 specimens and aside from any field observations on their geological relations, 

 as granite-porphyry or mica-granite, and probably of Paleozoic or early 

 Mesozoic age, from their resemblance to well-known rocks of that age in 

 other parts of the world. The hornblende-poi-phyrites, on the other hand, 

 might from the same standpoint have been classed as Tertiary andesltes. 



Orthociastic and piagioc'astic rocks. — The uow uuiversallv adopted cliemico- 

 mineralogical classification (based on Tschermak's classical studies) of ortho- 

 clastic and plagioclastic rocks is one which presents ever-increasing' difficul- 

 ties of application with the progress of nncroscopical and chemical investi- 

 gation. In the present instance the older rock series contain relative pro- 

 portions of orthoclase and plagioclase feldspar, often so evenly balanced 

 that the slight variations in their proportions, which may be found in differ- 

 ent parts of what is apparently the same mass, would be sufficient to justify 

 the placing of the same rock now in the orthociastic division now in the plagio- 

 clastic. Again, in those porphyries in whicli the orthociastic feldspars have 

 developed in large individuals, it is evident that so much orthociastic mate- 

 rial has thus been abstracted from the groundmassthat, were the latter taken 



