BIRCH HILLS. 



163 



porphyritic, the phenociysts being larger and fewer. They are: Oligoclase 

 in fine idiomorphic crystals, with marked zonal structure and little polysyn- 

 thetic twinning, sometimes suggesting orthoclase; biotite, in thick idiomor- 

 phic crystals, with numerous magnetite inclusions; and corroded quartz, 

 with glass inclusions. The groundmass is microgranular with idiomorphic 

 quartzes, the average grain being about 0.02 mm. in diameter. It consists 

 of quartz and feldspar, with small amounts of magnetite and biotite. Apatite 

 occurs in comparatively large colorless crystals, much cracked. Zircon is 

 present in minute prismatic crystals. The chemical composition of the rock 

 is shown in the following analysis, and is nearly identical with that of the 

 Bunsen Peak rock: 



Analysis of docile-porphyry of the Birch Hills. 



[J. E. Whitfield, Analyst.] 



Constituent. 



Per cent. 



SiO, 



70.24 



Trace. 



17.36 



1.38 



.79 



None. 



.53 



2.74 



3.69 



2.65 



None. 



Trace. 



Trace. 



.71 



Ti'Oa 



A1 2 3 



Fe 2 3 ... 



FeO 



MnO 



MgO .. 



CaO 



Na.O 



KO 



Li 



P.,0 5 



SO, 



H,0 



Total 



100. 09 





Eastward this rock passes beneath the rhyolites, which reach almost 

 to the summit of the hills. On the westward slopes steeply upturned 

 limestones, somewhat altered, dip toward the igneous rock. The foot slopes 

 show a remnant of the basic breccias covering the eroded limestones and 

 porphyry, and covered in turn by the ubiquitous rhyolite. 



The west spur of the northern hill is formed of altered limestones, 

 whose beds strike S. 40° E., and dip 70° NE., toward the dacite-porphyry. 

 The limestone is dense, gray, mottled with yellow, and underlies thinly 



