BASALT. 387 



with low extinction angles, which may most likely belong to a less basic feldspar. 

 The long narrow crystals are without zonal structure and are free from inclusions of any 

 kind. In several thin sections of somewhat more coarsely crystalline structure tin- 

 shorter, thicker crystals have both zonal structure and numerous globulitic glass inclu- 

 sions. 



The pyroxene constituent consists of both augite and hypersthene. The angitc in 

 thin section is almost colorless with a slight tinge of yellowish green. The hyper 

 sthene is colored light green and light reddish brown with the same pleocbroism as that 

 already noticed in the andesites, a phenomenon more common in the larger crystals, 

 though not of constant occurrence in any one thin section and frequently confined to 

 the inner portion of a crystal. The hypersthene is of older growth than the augite, 

 which frequently incloses slender prisms of the former. The crystals of angitc arc 

 not sharply outlined, except in a few of the larger individuals, but have an uneven, 

 jagged outline and are in the form of irregularly terminated prisms and grains, with 

 an octagonal cross-section, which is well defined in many cases, with the pinacoidal 

 faces more highly developed than the prismatic. It has a good cleavage parallel to 

 the latter, with an occasional less perfect jointing parallel to the former; there are also 

 irregular transverse fractures across the long slender prisms. The larger crystals are 

 sometimes twinned one or more times in the ordinary manner parallel to the orthopin- 

 acoid, and are often rich in glass inclusions with a g;is bubble and sometimes a color- 

 less microlite; apatite needles are not met with, but grains of magnetite are abundant. 

 A curiously curved crystal of augite occurs in thin section 260, one half being bent 

 without fracture through an angle of 40. Augite is the most abundant mineral com- 

 posing these basalts and is considerably in excess of the feldspar: the size of its grains 

 is variable, the majority ranging from 0-01 to 0-05"""; many are smaller and a large 

 number evenly scattered through the groundmass average 0-1 """ in diameter, while a 

 small number of porphyritically developed crystals measure 0-75""" in length and are 

 frequently associated in groups of half a dozen or more. Augite is also found in 

 aggregates of radiating prisms encircling macroscopic grains of quart/. 1 1 is in nearly 

 every instance perfectly fresh, but in thin section 202 a fibration parallel to the verti- 

 cal axis has taken place, accompanied by a red coloration around the margin of the 

 crystal ; the fibers polarize brilliantly between crossed nicols and extinguish light 



parallel to their length. Bancroft Libratf 



The olivine, which appears to be only locally developed in this group of basalts 

 and is found in only a few thin sections, is in porphyritical crystals and fragments, 

 the largest not more than 0-7 ranl long and some as small as (M5"" n . The sections are 

 in symmetrical figures of four and six sides, and also in irregular shapes; the outline 

 is not sharply defined, but notched. The substance of the olivine is eolorless in thin 

 section and very pure. There are in most cases t\vo or more straight cracks parallel 

 to the plane of the optic axes and the usual cleavage, besides numerous fractures in 



