MUSEUM OF COMPAKATIVE ZOOLOGY. 22'J 



the presence of CI, Xa, SO3, and a little Ca ; so that there is a mixture 

 of the hauynite molecule. The speciHc gravity of an isolated piece was 

 2.19, which is low for sodalite (2.28), and must be attributed to zeoliti- 

 zation. W. Lindgreu has described,^ under the name " analcite-basalts," 

 basic rocks from tlie Highwoods (a range lying north of the Crazy 

 Mountains and a locus of similar magmas), composed of olivine, mag- 

 netite, a little biotite, and a mmeral determined as analcito, but no feld- 

 spar or nepheline. The analcite appears in the slides in hexagonal or 

 octagonal sections, clear and isotropic, and was determined to be such 

 by specific gravity and chemical analysis. Lindgren considers this pri- 

 mary. Judging from the descriptions, there is some resemblance to the 

 mineral identified by us in the present rock, without quantitative anal- 

 ysis, as sodalite ; but as the latter has the form and other properties of 

 sodalite, and is identical with that mineral as occurring in the granular 

 and porphyritic theralite, this seems the correct determination. 



Aurjite. — Tlie porpl)yritic augite has planes developed in the pris- 

 matic zone, sometimes terminal, and the common orthopinacoidal twin- 

 ning. In sections the pale green of the centre of the crystals gradually 

 increases in deiitli towards tlie edge, which is formed of deep green 

 feo-irine, as in the theralites, havinii- the characteristic small obliquity of 

 extinction in the prismatic zone and axis of elasticit}', n, near the vertical 

 axis. The augite is generally fresh, and includes crystals of magnetite 

 and sodalite. 



The slides contain occasional plates of biotite and grains of magnetite. 



Groiindmass of the Acmite-trachi/le Type. — This is composed essen- 

 tially of slender lathe-shaped feldspars and acicular crystals of a^girine, 

 whifli lie either between the feldspars or included in them, and by 

 preference in the outer poi'tion. These minerals appear in sections 

 transverse to the flow with perfect fluidal arrangement ; but when cut 

 parallel, many of the feldspar sections are broad and rounded, and contain 

 the a'girine needles in a network without parallel arrangement. The 

 feldspars are therefore somewhat tabular in the plane of flow. Single 

 crystals, Carlsbad twinns, or multiply twinned crystals occur, with ex- 

 tinctions parallel or but slightly olilique to their length, and are doubt- 

 less also anortlioclase in part. With the green a'girine a few brown 

 needles of acmite occui\ Apatite and magnetite are found in the usual 

 form. The combination of nearly parallel feldspar and ajgirine needles 

 gives the former a peculiar feathery look in polarized light with low 



1 " Eruptive Kocks from Montana," Proc. Cal. Acad. Sci., Vul III. p. 51. 



