WRIGHT-FENNER] PETEOGRAPHIC STUDY 87 



a rule, remarkably fresh and unaltered. The amount of volcanic 

 glass present varies within wide limits. Specimens 263743 and 

 263747 consist almost entirely of colorless volcanic glass which has 

 many of the characteristics of tuffaceous material. The glass frag- 

 ments are splintery and irregular and often contain streaks of elon- 

 gated bubbles. The refractive index of the glass and its chemical 

 composition are fairly constant in each specimen; in 263743 the refrac- 

 tive index averages about 1.500, in 263747 about 1.485. These re- 

 fractive indices are both low and indicate a high silica content, speci- 

 men 263747 being slightly more siliceous than 263743. In other loess 

 specimens the glass is less abundant and may even become rare. In 

 all cases it is colorless or nearly so and of very low refractive index. 

 In contrast to this the glass in the scoriae is usually colored and of 

 higher but variable refractive index, which ranges from about 1.51 

 to 1.56 and above in some of the specimens. 



The mineral fragments in the loess are usually angular and aver- 

 age about 0.1 mm. in length. They occur almost invariably in single 

 isolated grains and not in crystalline aggregates nor embedded in a 

 groundmass. No fragments of an eruptive rock were observed with 

 certainty. 



The quartz grains are irregular in shape, and rarely, if ever, show 

 crystal outlines. They are of normal quartz which has suffered 

 little deformation and may be in part eruptive quartz and in 

 part vein quartz. The plagioclase occurs in angular, broken indi- 

 viduals twinned after the albite law and rarely after the Karlsbad 

 and pericline laws. The composition of the plagioclase is not the 

 same in the different specimens and often varies within wide limits 

 in the same specimen. In the one its composition may range from 

 albite-oligoclase to andesine; in another, from andesine to basic 

 labradorite. Zonal structure is not uncommon, but is by no means 

 the rule. The pyroxene crystals are prismatic in form, generally 

 pale-green in color and unusually fresh and free from alteration. 

 They are ordinarily of the diopside-augite varieties and are more 

 abundant than the green hornblende grains which occur in many but 

 not all of the loess specimens. The magnetite is relatively abundant 

 and occurs in irregular, more or less ecpiant, grains and octahedra. 

 The other minerals noted in the descriptions above are less common 

 and less constant in their occurrence. The relatively fresh, unal- 

 tered state of practically all the grains is noteworthy. They are 

 often covered with fine, cryptocrystalline, argillaceous material, 

 but do not appear to have altered into it to any appreciable degree 

 in most cases. 



Cliemical evidence. — The predominance of eruptive minerals in 

 the loess is clearly expressed in the chemical analyses, which might 

 easily be mistaken for tliose of eruptive rocks, were the water content 

 a little lower. 



