DIORITE. 333 



mass of the rock, carefully avoidiug tlie large piuk crystals, with the result of 2.95 

 per ceut. of potash aud 2.61 per ceut. of soda. As small flakes were used for this 

 purpose, it is probable that the grouudmass was present iu abnormal quantity, thus 

 cau.siug a relative increase in potash, even while excluding the large orthoclase crys- 

 tals. Plagioclase was found to be much subordinate in the groundmass,as stated above. 

 The large piuk orthoclase crystals themselves were then analyzed, with the result: 



SiOj 62.22 



AlnO., 20.33 



CaO 2.95 



K,0 B.31 



Na-jO 3.45 



LiiO Trace 



Igii 1.90 



Loss 84 



100. 00 



Careful examination of the material used showed only a few si^ecks of biotite, but 

 some soda-lime feldspar must have been present, judging from the large amount of 

 lime found. A determination in another clear crystal chosen for its apparent purity 

 gave nearly 3 per ceut. of lime again. The loss is thought by the analyst, Mr. Hille- 

 brand, to be chiefly soda. 



DIORITE. 



Of the distinctly plagioclastic rocks of the region but very few arc granular in 

 structure, the great ma;iority being diorite-porphyries, or porphyrites, as they will here- 

 after be designated. The three granular diorites found, rej>resent three very distinct 

 varieties, one of them being the only pyroxene-bearing rock occurring within the area 

 of the map. All occur, moreover, iu the same gulch, aud quite uear each other. 



QUARTZ-MICA-DIORITE. 



This rock occurs on the south side of Buckskin gulch. Park Countj", as a broad 

 dike, forming for some distance the southeast wall of one of the elevated amphitheaters 

 on Loveland Hill, and thence projecting as a knoll into the gulch opi>osite the lied 

 amphitheater. It disappears under loose material before reaching the stream bed, and 

 no continuation of the dike on the north side of the gulch was observed. The rock 

 has a fine, evenly-grained structure, with feldspar and quartz strongly predo:uinat- 

 ing over the small, irregular leaves of biotite. 



Microscopical examination shows zircon, magnetite, apatite, biotite, plagioclase, 

 orthoclase, and quartz as original constituents. None of the essential minerals is 

 well developed in crystal form and none shows noteworthy peculiarities. Plagioclase 

 is largely in excess over the orthoclase and quartz is quite abundant. All are quite 

 fresh, the biotite alone showing incipient decompositiou [117]. 



HORNBLENDE-DICKITE. 



A broad dike crossing the head of Buckskin gulch from Democrat Mountain in 

 a nearly eastand-west direction was found to consist of a very simple, normal diorite 



