WRIGHT-FENNBR] PETROGBAPHIC STUDY 57 



series of experiments, however, larger charges (about 100 grams) 

 of material were heated in a Fletcher gas furnace and with them 

 the oxidation was found to be much less, the center of the charges 

 containing magnetite and resembling the scoriae more closely than 

 did the smaller charges. The dependence of the product obtained 

 on the mode of heat treatment to which it had been subjected, 

 was thus clearly demonstrated, and is a factor which should be 

 taken into account in all such work. 



Descriptions of Specimens 

 specimens from buenos aires 



No. 263702. Specimen label. "Samples of the Pampean forma- 

 tion (brown and green) from excavation on Veinte Cinco de Mayo, 

 Buenos Aires ^ 



Under this number two distinctly different earths are included; 

 the first, a light-brown, porous earth of finef even grain, the second, 

 a pale ashy-green earth, mottled in appearance and noticeably dif- 

 ferent from the first in composition. Under the microscope the 

 powder of the brown eai-tli was found to consist largely of fragments 

 of plagioclase feldspars, glass, and fine earthy material, the last 

 usually stained with iron oxide. The plagioclase feldspars range in 

 composition from oligoclase to labradorite and often show zonal 

 structure. The glass particles are usually colorless and angular or 

 splinteiy in outline and are relatively fresh. Occasional grains of 

 pyroxene, hornblende, biotite, quartz, sanidine, apatite, zircon, and 

 magnetite were also observed and are all fairly fresh. Most of these 

 minerals and the glass are of igneous origin and have evidently been 

 assembled from various types of igneous rocks, the majority of which 

 were probably andesitic or dioritic in character. 



The second type of earth consists largely of fine calcite with 

 occasional larger grains of plagioclase, quartz, and volcanic glass. 

 Some argillaceous material is apparently also present. The plagioclase 

 is usually fresh and often exhibits zonal structure. This earth 

 differs from the first chiefly in its high calcium carbonate content. 



On immersion in water the brown earth in this specimen softens 

 and crumbles into brown mud, while the gray earth remains intact 

 and hard. On testing the water solution above the earth with silver 

 nitrate, only a trace of chlorine was detected. 



No. 263747. Specimen label. "Locality: Buenos Aires, on the old 

 Moreno place, where an elevation that rises above the general level of the 

 plain is being graded down. Material: White earth which occurred in 

 lenticular layer's 1 inch to 3 feet thick in the brown loess that looks like 

 upper Pampean. Probably volcanic ash." 



