WRIGHT-FBNNER] PETROGBAPHIC STUDY 59 



No. 263716. Specimen label. ^'Locality: Alvear, on the Parana. 

 Material: Loess selected bij Santiago Roth to ref^resent the Middle Pam- 

 pean, which he regards as younger than the deposit in the bottom of the 

 ravine. B. W. 26 June, 1910." 



In general appearance this specimen is practically identical with 

 the preceding. It is a light-gray, even-grained earth, coherent but 

 not indurated, which consists microscopically of plagioclase, quartz, 

 glass, and argillaceous material. Minor components are hornblende, 

 pyroxene, magnetite, and spinel (or garnet ( ?)) . As in the preceding 

 specimens, igneous and volcanic materials constitute the major part 

 of the specimen. Both this earth and the preceding disintegrate 

 when immersed in water. Treated with silver nitrate, they give a 

 slight chloride reaction. 



This specimen of earth, which is practically identical with 263715, 

 was moistened and heated to 510°. The material was caked but not 

 reddened by this treatment. At 710° it was slightly reddened. 

 Held at 1,000° for 4f hours, the color became a bright brick-red and 

 the material was considerably indurated, but no sign of fusion was 

 observed under the microscope. This material was noticeably like 

 some of the tierras cocidas, but was not so hard. 



No. 263717. Specimen label. ^'Locality: Alvear, on the Parana. 

 Material: Loess from the Middle Pampean of Roth, taken from the hanJc 

 3 feet from, and on the level of, the hurnt clay. B. W. 26 June, 1910." 



A light-gray, rather compact, earth of fine, even grain. Similar in 

 texture and general appearance to the two specimens just described 

 from the same localit}^ Microscopically, this specimen exhibits the 

 characteristic minerals of many of the loess specimens of this region. 

 Plagioclase, quartz, and glass, and argillaceous material constitute 

 the bulk of the rock, while occasional grains of hornblende, pyroxene, 

 biotite, and magnetite are scattered through the specimen. 



This earth disintegrates partially to a lumpy aggregate when 

 immersed in water. Its water solution gave only a slight trace of 

 chloride when tested with silver nitrate. 



Fragments of this specimen heated to 870° for 16 hours became 

 bright brick-red and indurated and could not be distijiguished from 

 some of the fragments of 263729. Heated to 1,200° for 3 hours, the 

 earth was fused to a dark-purple mass, in which fragments of the 

 original minerals appeared under the microscope, embedded in a 

 colorless to pale-brown glass. The colorless glass was filled with 

 minute specks of iron ore, while the pale-brown glass was relatively 

 free from such specks and had a slightly higher refractive index 

 (n = about 1.530) than the former (n = about 1.510). 



Chemical analysis. — Although this loess contains argillaceous 

 material and can not be constant in its composition, it does contain 



