WRIGHT-FENNBR] PETEOGEAPHIC STUDY 83 



mineral of weaker birefringence and refractive index about 1.57 

 occurred in overlapping aggregates which were too fine for satisfac- 

 tory determination but may have been plagioclase. Some glass with 

 refractive index about 1.525 was also present. 



No. 263707. Specimen label. ^'Locality: Monte Hermoso near 

 the old lighthouse on the coast east of Bahia Blanca. Material: Loess- 

 sand of the Monte Hermoso formation from exposure above liigh tide." 



A light brownish-gray, firm, but distinctly sandy, material consist- 

 ing of plagioclase, quartz, and colorless glass. The plagioclase is 

 occasionally fairly fresh but more often slightly decomposed and 

 stained with iron oxide. Minor components are hornblende, pyrox- 

 ene, apatite, and magnetite. The grains are generally of fair size 

 and rounded from transportation. Scattered through the rock are 

 minute needles and cryptocrystalline material which is too fine to 

 identify with certainty. It is in part evidently argillaceous in char- 

 acter. When immersed in water this earth crumbles into a lumpy, 

 sandy mud. Tested with silver nitrate it gives a distinct chloride 

 reaction. 



No. 263714. Specimen label. '^Locality: Monte Hermoso near 

 the old lighthouse. Material: Yellow-brown loess of the Monte Hermoso 

 formation from the upper bench 5 feet above tide." 



A light-brown, porous earth, fine and even-grained and consisting 

 in large measure of argillaceous material with some plagioclase, 

 volcanic glass, and occasional grains of pyroxene arid biotite. On 

 making a concentration test with the powder of this specimen, pla- 

 gioclase of different compositions, quartz, pyroxene, hornblende, bio- 

 tite, apatite, zircon, magnetite, spinel (or garnet), and epidote were 

 observed; also possibly olivine and monazite and two other minerals 

 which were not identified. Most of these minerals are of igneous 

 origin but several are of metamorphic origin. In water this earth 

 disintegrates into a lumpy clay. Its water solution gives a strong 

 chloride reaction when treated mth silver nitrate solution. 



This earth, heated to 1,200° for 1 hour, fused to a brown glass in 

 which many of the original crystal fragments of quartz and plagioclase 

 were still visible. Hematite in minute crystals was found to have 

 crystallized from the melt and at certain points to be so abundant as 

 to render the glass nearly opaque. The refractive index of the glass 

 ranged from 1.525 to 1.54. On heating the earth to 700° for 45 

 minutes its color was found to have changed from brown to brown- 

 red. 



No. 263741. Specimen label. "Locality: Monte Hermoso near 

 the old lighthouse. Material: Basal layer of cross-stratified sands 

 (the Puelchean) upon Monte Hermoso formation." 



An indurated, light-brown material composed apparently of a 

 mixture of clay and quite coarse sand through which numerous 



