282 IRVING. 



biotite seems to have been one of the most resistant minerals, and 

 remains protruding in ghttering flakes, which gives to the 

 weathered specimens a very basic appearance. 



Microscopic Characters. — The microscope shows the follow- 

 ing minerals : plagioclase, orthoclase, biotite and hornblende 

 or chlorite, as phenocrysts ; plagioclase, magnetite, chlorite, 

 calcite and quartz in the groundmass. The plagioclase pheno- 

 crysts show the usual polysynthetic twinning. They are gen- 

 erally fairly fresh in the central portions, but are somewhat 

 decomposed along their borders. Measurements made in sec- 

 tions on the zone perpendicular to J/ showed maximum extinc- 

 tion angles of 23 degrees. This would place the feldspar among 

 the oligoclase-andesines. 



The orthoclase phenocrysts vary in abundance, but are al- 

 ways subordinate to the plagioclase. 



In the rock from the gulch west of Labrador, orthoclase is 

 entirely absent, both as phenocrysts and in the groundmass. In 

 that from the Rua Mine it appears as occasional phenocrysts, 

 and from the Redpath Creek laccolite it equals the plagioclase in 

 amount. The biotite is in hexagonal and rectangular flakes and 

 usually quite fresh. It has a strong pleochroism, opaque 

 brown, parallel to the cleavage, and greenish brown at right 

 angles to it. The absorption is much stronger in the larger and 

 fresher masses. In those which show a slightly decomposed 

 border it is colorless parallel to the cleavage, and light yellow 

 at right angles to it. 



The hornblende was observed in a fresh condition in only one 

 specimen, that from the flat northwest of Twin Peaks. Here 

 it occurs in large crystals with blunt terminations, showing 

 cross sections }^ x^i inches in maximum development. The 

 crystals are short and thick set, never acicular, and show the 

 usual cleavage characteristic of the mineral. They have the 



faces CO P 00 (010), 00 /• (no) in strong development, and are 

 terminated by the usual Poo (on)- No other faces were observed 

 Maximum extinction angles range about 12° to 15°. 



