WRIGHT-FENNEE] PETEOGKAPHIC STUDY 65 



the doubtful minerals are also at home in metamorphic rocks or in 

 veins rather than in igneous rocks. 



On immersion in water this earth does not break down easily. 

 The water solution above it was found to contain abundant cldoride 

 in the form probably of sodium chloride derived from salt water. 



On heating this earth to 1,100° for 30 minutes, a hard, fused lump 

 with glazed surface was obtained, purple in color and containing 

 microscopic fragments of quartz, plagioclase, pyroxene, and mag- 

 netite embedded m a nearly colorless glass base, wliich was often 

 clouded by innumerable specks of iron oxide. On heating another 

 portion to 1,200° for 3 hours and then at 1,150° for 3^ hours, the 

 melt was found to have settled in the crucible to a dark purphsh- 

 brown glass, which still showed under the microscope fragments of 

 the original minerals. The glass in this case, however, was notice- 

 ably brown in transmitted light and contained imiumerable minute 

 hematite scales which had evidently crystalhzed from the glass; no 

 evidence of other secondary crystals was obtained. The colorless 

 glass (n = 1.50) has a lower refractive index than the brown-colored 

 glass (w= 1.515 to 1.525). 



No. 263713. Specimen label. ''Locality: Barrancas del Norte, 

 about If. miles north of Mar del Plata. Material: Samples of pehhly 

 loess, filling Jiollows in the green and hrown loess beneath. Formation: 

 Ameghino's ' Bonaerean.' " 



A light brownish-gray, fine-grained earth containing many rounded 

 fragments wliich are of similar material, but sUghtly more indurated. 

 Under the microscope the matrix was found to consist largely of 

 argillaceous material usually stamed with iron oxide. Plagioclase 

 and glass are also abundant. Hornblende, pyroxene, epidote, and 

 magnetite are present in subordinate quantity. The pebblehke 

 fragments consist of practically the same substances. In them 

 pyroxene seemed slightly more abundant, and spinel and apatite 

 were observed in addition to the minerals noted in the matrix. This 

 earth disintegrates in water to a lumpy mud. Its water solution 

 contains abundant chloride (probably common salt from sea water). 



No. 263706. Specimen label. "Locality: Barrancas del Norte, 

 north of Mar del Plata, 600 feet south of Arroyo Came. Material- 

 Loess from stratum in which wind-hollowed pocJcet was excavated, 4 

 feet above high tide." 



A fairly hard, grayish-brown earth, consisting in large part (60 per 

 cent or more) of isotropic, colorless glass, which appears in angular 

 sphnters or in rodlike forms produced evidently by the drawing out 

 of viscous glass. Gas inclusions in the shape of fine capillary tubes 

 are characteristic of such glass rods. Besides the volcanic glass 

 much argillaceous material is present ; also fragments of plagioclase, 

 21535°— Bull. 52—12 5 



