66 BUREAU OF AMERICAN ETHNOLOGY [bull. 52 



quartz, hornblende, biotite, pyroxene (e^ddently several varieties in 

 shades of brown, green, and purple), magnetite, apatite, zircon, 

 tourmaline, epidote, and probably spinel. Except for the larger 

 proportion of volcanic glass this specimen is similar to specimen 

 263704. 



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

 water solution surrounding the earth gives a strong cliloride reaction 

 when treated vnth silver nitrate. 



A portion of tliis earth was mixed with grapliite and heated to 

 1,300° for a few minutes and then held at 1,100° ±30° overnight 

 (16 hours). A pale greenish glass resulted, which contained isolated 

 remnants of the original quartz and plagioclase fragments. Hex- 

 agonal crystals or aggregates of hematite, which had been formed 

 from the melt, occurred frequently and were the only evidence of 

 crystalhzation from the magma. The refractive index of the glass 

 was found to vary considerably (from 1.50 to 1.525). 



No. 263708. Specimen label. "Locality: Barrancas del Norte, 

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

 Filling of wind-hollowed cavity, talcen 4 feet above high tide." 



A pale-brown, fine earth containing microscopic fragments of 

 plagioclase (andesine and labradorite, the individual crystals often 

 showing zonal structure), volcanic glass, quartz, sanidine, hornblende, 

 biotite, pyroxene, magnetite, and zircon. The })lagioclase is fairly 

 abundant and fresh. Argillaceous substance, often stained with 

 iron oxide, is scattered through the specimen and coats the indi- 

 vidual mineral grains. In tliis specimen, as in most of the pre- 

 cedmg, the average diameter of the grains is about 0.1 mm. Com- 

 pared with specimen 263706 tliis specimen contains noticeably less 

 volcanic glass. 



When immersed in water this earth crumbles somewhat, but not 

 entirely, many large resistant lumps remaining intact. The water 

 solution above it gives a strong chloride reaction when treated with 

 silver nitrate. 



No. 263711. Specimen label. "Locality: Beach l}/2 miles north 

 of Mar del Plata. Material: Pehhly loess resembling in the bed ei soft 

 volceinic agglomerate. Formeition: ' Ensenadean' of Ameghino." 



The present hand specimen gives practically no hint of the pebbly 

 character of this formation, but appears to be a uniform gray-browai 

 earth, even grained, and fairly coherent. Under the microscope the 

 principal components are plagioclase and glass, both much decom- 

 posed. Hornblende, biotite, pyroxene, magnetite, and epidote were 

 also observed in scattered grains. In general aspect and composition 

 this earth is not greatly different from those precedmg. Ai-gillaceous 

 substance is abundant and is evidently due in part to weathering. 



