64 BUREAU OF AMERICAN ETHNOLOGY [bull. 52 



a chemical analysis or for detailed tests. It appears to have resulted 

 from the interaction of calcium carbonate- and calcium phosphate- 

 bearing solutions, the calcium phosphate having been derived from 

 the bones which appear in this stratigraphic horizon. 



SPECIMENS FROM THE VICINITY OF MAR DEL PLATA 



No. 263704. Specimen label. '^Locality: Four miles north of Mar 

 del Plata in the Barrancas del Norte. Material: Greenish loess form- 

 ing a stratum near high tide. Ameghino's Belgranean." 



A pale greenish-gray, pulverulent material consisting in large part 

 of plagioclase, glass, and argillaceous substance. With this specimen 

 a special concentration test was made to determine as completely as 

 possible the minerals present, for the purpose of ascertaining whether 

 any mineral of other than igneous origin occurred. A considerable 

 quantity of material was first washed with water and the clay and 

 major part of the lighter minerals thus removed. The remaining part 

 was then separated in a heavy Thoulet solution and a heavy, dark- 

 colored, residual sand obtained which consisted of a great variety of 

 minerals. Inclusive of the lighter constituents the following minerals 

 were recognized : (1 ) Plagioclase (andesine, and labradorite, the grains 

 often showing zonal structure); (2) quartz; (3) sanidine; (4) pyroxene 

 (diopside, augite, and titaniferous augite); (5) hornblende; (6) glass, 

 colorless and brown. Many of the characteristics of volcanic glass 

 are clearly shown in this specimen. The particles often appear as 

 elongated rods and frequently contain long-drawn-out gas inclusions. 

 Splintery and pronglike forms are not uncommon and often show 

 conchoidal fracture. Birefringent spherulites occur in certain 

 instances and practicaUy replace the volcanic glass. The glass does 

 not appear to have suffered much attrition, but the individual grains 

 are too small to be affected seriously by water transport. That this 

 volcanic glass does not have the same composition throughout is 

 evident from the variation from grain to grain in the refractive index, 

 which ranges from about 1.50 to 1.54. Evidently material from 

 different sources has been brought together by water action. (7) 

 Magnetite; (8) biotite; (9) and (10) epidote and zoisite in consider- 

 able quantity and of various types; (11) zircon; (12) tourmaline; 

 (13) titanite; (14) apatite; (15) garnet; (16) spinel (probable); (17) 

 brucite (probable); (18-22) five minerals which could not be posi- 

 tively identified. One agreed well in its optical properties with 

 monazite; another was suspected to be wulfenite. 



Most of these minerals are typical of igneous roclvs, but a few of 

 them, as epidote and zoisite, which are abundant, are metamorphic 

 rather than igneous minerals. Tourmaline is a common contact 

 mineral, but occurs also in certain kinds of igneous rocks. Some of 



