MISCELLANEOUS POEPHYEITES. 343 



certain clew. Au alkali determination in one of the freshest specimens [109] yielded 

 Xa 2 O 4.08 per cent, and K 2 O 2.70 per cent., which must be decisive in confirming the pres- 

 ent classification, for it is to be expected from analogy with the fresher rocks previously 

 described that the larger part of the soda will be contained in the porphyritic crys- 

 tals of the first generation, while the potash will remain chiefly in the groundmass. 

 When nearly balanced alkalies are, as a matter of fact, so disposed in the solid rock, 

 the soda feldspar certainly becomes the more prominent and should determine the 

 classification. 



The groundmass is holocrystalline, in most cases coarsely macrocrystalline, and 

 is made up of quartz, orthoclase, and plagioclase, with some biotite. Its constitution 

 is often obscured by chlorite. The amount of quartz seems less than in most porphy- 

 rites described, and a silica determination gave but 60.42 per cent. [109&]. Epidote 

 and chlorite are the products of the decomposition of both biotite and hornblende. 

 What seem at first sight to be included fragments of arnphibolite are most probably 

 Accretions of hornblende from the magma in an earlier period of the rock's history. 

 Biotite and some feldspar accompany the hornblende. 



On the extreme southern spurs of Mount Silverheels, beyond the limits of the 

 present map, between the forks of Crooked Creek, a variety of much more distinct 

 porphyritic habit was found, which is colored on the Hayden map as "Porphyritic 

 Trachyte" [108]. Its crystals reach l om in diameter and predominate over the light- 

 grayish groundmass. All the elements are the same in character as in the Silver- 

 heels rock, and it can be regarded only as a modification of the same. 



MISCELLANEOUS PORPHYEITES. 



The "Green Porphyry," a peculiar fine-grained rock, was found occurring in three 

 different places: first, as a dike, running from the northern edge of Bross amphithea- 

 ter toward Mount Silverheels [98a]; secondly, on the north side of Mosquito gulch, 

 near its mouth, interbedded in Cambrian quartzites [98]; and thirdly, as an interstrat- 

 ified bed on lower Loveland Hill, near the Fanny Barrett and Eagle Bird claims 

 [986]. It is macroscopically compact, light gVeen in color, with an abundant chloritic 

 decomposition product, which renders it difficult to distinguish clearly each crystal indi- 

 vidual, although it is sometimes plain that the rock is almost wholly macrocrystalline. 

 Quartz, feldspar, biotite, and hornblende are, however, recognizable, the latter two 

 being much altered. 



Some of the thin sections prepared show no normal groundmass at all, although 

 a distinction can be made between certain well-crystallized elements and wholly irreg- 

 ular fragments. There seem to have been original crystals of feldspar, hornblende, 

 and biotite, all quite small, while the remainder of the rock, solidifying later, was 

 formed of the same minerals, with quartz, in irregular grains, which sometimes have 

 reached the size of the crystals, but more frequently have not. 



The feldspars are largely replaced by muscovite and calcite ; the dark silicates 

 by chlorite and epidote. Quartz is not abundant, a silica determination yielding but 

 63.85 per cent. A few fluid inclusions are observed in quartz and feldspar. 



In connection with the above rocks should be mentioned several occurrences not 

 to be classed under any of the described varieties, though most closely allied to the 



