HASALT. 3ST 



with low extinction anjijles, wliicli may most likely helonj^ to a less basic- fel(lsi)ai'. 

 The long narrow crystals are without zonal stiMK-tureand are free from inclusions of any 

 kind. In several thin sections of somewhat more coarsely crystalline stiucture the 

 shorter, thicker crystals have both zonal structure and numerous yloljulitic jj^lass inclu 

 sions. 



The pyroxene constituent consists of both augite and hypersthene. The anj;ite 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 pleochroism as that 

 already iu)ticed in tlie andi^sites, a phenomenon more common in the larger crystals, 

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

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

 which freijncntly incloses slender ])risnis of the former. The crystals of augite are 

 not sharply outlined, excei)t in a few of the larger individuals, but have an uncvc!!, 

 jagged outline and are in the form t)f irregularly terminated prisms and grains, with 

 an Oi'tagonal 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 jtarallel to the former; there are also 

 irregular transverse fractures a(Toss the long slender prisms. The larger crystals are 

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

 acoid, and are often rich in glass inclusions with a gas bubble and sometimes a color- 

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

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

 without fracture through an angle of 4()o. Augite is the most abundant mineral-com- 

 posing these basalts and is considerably in excess of the feldspar: the size of its gi-aiiis 

 is variable, the majority i-anging from O-Ol toO-0.')"""; many are smaller and a large 

 number evenly scattered through the groundmass avei'age 0-1 """ in diameter, while a 

 small iiund)er of porphyritically developed (-rystals 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 quartz. 1 1 is in nearly 

 every instance perfec-tly fresh, but in thin section 2(i2 a fibration jiarallel to the \-erti- 

 cal axis has taken place, accompanied by a red coloration an)und the margin of the 

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

 l)arallel to their l»^ngth. 



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

 and is timnd in only a few thin sections, is in porphyritical crystals and fragnuMits, 

 the largest not more than Ow""" long and some as small as ()-05""". The sections are 

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

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

 section and very pure. There are in most cases two or more straight cracks parallel 

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



