RESUME. 355 



THE KOCK STRUCTURES OBSERVED. 



But three granular rocks were found, all of tbeui diorites. In striking con- 

 trast to this rarity, it is observed that all the numerous quartz-porphyries and por- 

 phyrites are holocrystalline and that the groundiuass is in nearly all cases evenly 

 granular. Although these rocks occur both in dikes and in relatively large masses, 

 this markedly crystalline structure is wonderfully persistent through the extent of 

 the existing variation in conditions. 



INDIVIDUAL ROCK TYPES. 



Of the various rock types described, the following seem specially noteworthy: 



1. White Porphyry — This rock illustrates a transition stage between the granu- 

 lar and porphyritic structures. Its imbedded crystals are few and small, but they 

 evidently correspond to the more prominent constituents of the typical porphyry, 

 whether viewed from the structural standpoint or considered in the light of the genetic 

 principle discussed in the introduction. In mineralogical composition this is an inter- 

 esting type, because of the absence of biotite or a bisilicate as an essential constitu- 

 ent. Even the intimate relationship to a biotite bearing rock indicates nothing more 

 than the possible presence of biotite in very insignificant quantity. The common acces- 

 sories, apatite and magnetite, are also very rare. 



2. Lincoln Porphyry — This widely distributed type is remarkable for its large 

 orthoclase crystals, developed during the later stages of consolidation, in the presence of 

 abundant plagioclase. The persistency with which these crystals are found in masses 

 of various conditions of occurrence gives at first a somewhat erroneous impression as 

 to the distinctness of the type. Only the observance of many occurrences leads to a 

 correct understanding of the relations of this rock. 



3. Nevadite — This variety has solidified at a stage seldom illustrated by instances- 

 which have been previously described. In its granular groundmass, consisting almost 

 wholly of quartz and orthoclase, are still a few isolated particles of clear glass, a case' 

 directly analogous to but one occurrence known to the writer. The present forim 

 may be considered as a fair type of the division of the rhyolite called "Nevadite'"' 

 by Messrs. Hague and Iddings. The peculiar mineralogical components are referred 

 to below, and a glance at the quantitative analysis will show a wonderfully simple 

 chemical constitution. Silica, alumina, potash, and soda make up 97.67 per cent, 

 of the whole, no other element reaching 1 per cent. 



4. Hypersthene-bearing andesite — The rocksfrom the Buffalo Peaks, in which hyper- 

 sthene' was identified as a prominent constituent, are especially noteworthy only 

 as the first ones in America in which the important role played by that mineral was 

 recognized. The experience of the last two years has shown the writer that andesites 

 containing hypersthene as an essential constituent are very abundant in Southwestern 

 Colorado, wliile their distribution in the Great Basin and among the volcanoes of the 

 Pacific coast has been shown by the publications of Messrs. Hague and Iddings- and 

 Diller.^ 



' Bulletin No. 1, United States Geological Survey, 188:?. 



'American Journal of Science, III, XXVI, 222, lb83. Idem., XXVII, 453, 1884. 



'American Jonrnal of Science, III, XXVIII, 252, 18--'4. 



