262 IRVING. 



the series. Mention will be made under each mineral of the 

 varieties in which it is most common, and when it occurs only 

 in a single type a description of that rock will be included under 

 the discussion of the mineral. 



Ortlioclasc. — Although occasional crystals of albite occur 

 in the groundmass of the more basic members of the series, 

 the prevailing feldspar of these rocks is sanidine. Anortho- 

 clase, while probably present in considerable abundance has no- 

 where been identified with certainty. The orthoclase occurs in 

 large automorphic phenocrysts, and lath-shaped crystals, or 

 xenomorphic grains in the groundmass. The phenocrysts are 

 most frequently untwinned, showing the faces P, M, 1 and x, 

 more rarely y ; other faces were not observed. The pheno- 

 crysts contrast very markedly in habitat with those of many of 

 the quartz-porphyries. The latter are prevailingly square in 

 cross-section, showing as a rule only P and 1. 



Zeolitic alteration is quite often observed, and natrolite seems 

 to be the most common product, although secondary quartz 

 occurs quite frequently. In the majority of tinguaites the pheno- 

 crysts are small, being elongated parallel to a so that their length 

 is often twice their breadth. They are scattered here and there, 

 and attain a size of )/j^ inch in length. 



In two instances, however, they show a rather remarkably 

 large development, namely, in the dike east of the Rua mine 

 and in that northwest of the Twin peaks exposure. In the first- 

 named rock they are thickly scattered through a dense dark- 

 green groundmass and are oblong with a larger diameter often 

 exceeding one inch. 



These feldspars are unusually fresh and generally twinned 

 after the Carlsbad Law. They are surrounded by a decom- 

 posed rim in many cases, but are never zonally built. In the 

 rock from beyond the Twin peaks they are prevailingly larger 

 and more elongated, especially near the contacts of the dike 

 with the wall rocks. Resorption has generally been so slight 

 as to leave the crystals quite sharp, but fracture has not infre- 

 quently occurred in such manner tliat the groundmass has been 

 permitted to penetrate between the broken portions of the 

 crystal. 



