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 factors. 



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

 tion, forming only an accessory 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-porphyrites, on the other hand, 

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



Orthociastic and piagiociastic rocks. The now universally adopted chemico- 

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

 clastic and piagiociastic rocks is one which presents ever-increasing difficul- 

 ties of application with the progress of microscopical 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 orthoclastic division now in the piagio- 

 ciastic. Again, in those porphyries in which the orthoclastic feldspars have 

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

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



