80 BUREAU OF AMEEICAN ETHNOLOGY [bull. 52 



SPECIMENS FROM THE VICINITY OF NECOCHEA 



No. 263745. Specimen label. ''Locality: Playas near tJie sea in 

 the neighborhood of Laguna Malacara, north of Necochea. Material: 

 Scoriae J' 



No. 263746. Specimen label. "Locality: Coast north of Necochea, 

 heyond the Boca del Moro, Laguna Malacara vicinity . Material: Scoria 

 found on the surface of various playas where the Ensenadean is exposed.'' 



These two samples include a number of specimens which are prac- 

 tically identical in external appearance and in the thin section. In 

 the hand specimen they bear a close resemblance to normal lava 

 scoriae and might easily be taken for such, but under the micro- 

 scope they do not resemble any known type of lava. (PI. 8, a.) 

 Fragments of quartz, plagioclase, and pyroxene are set in a streaky- 

 brown glassy matrix out of which radial and irregular groups of 

 acicular pyroxene crystals and less frequently of plagioclase have 

 been precipitated. The pyroxene was determined optically by its 

 high extinction angles, high refractive index, fairly strong birefring- 

 ence, square-end section, and positive optical character. It is usually 

 colorless or pale-brown in color while the larger fragmental pyroxenes 

 are pale-green in color, have liigher refractive index, and are usually 

 rounded in outline. It is evident that not only did they not crys- 

 tallize out of the glass but they were unstable in it and were attacked 

 and corroded by it. The plagioclase microlites are lath-shaped 

 in section and apparently andesitic in composition. They proved 

 too fine for more exact determination. The glass varies in compo- 

 sition from point to point, as is evident from its streakiness and 

 the differences in color and refractive index (ranging from 1.515 to 

 about 1 .55) . Fi»e dusty particles, possibly magnetite, are abundant 

 in the glass and frequently become so abundant as to impair seriously 

 the transparency of the glass. The cavities of these scoriss are often 

 filled with rounded grains and pellets of quartz, plagioclase, magne- 

 tite, chert, and limestone. Some of these grains are tightly wedged 

 into the cavities and have evidently been lodged there either by 

 water or wind action, possibly during the transport of the scoriae. 



Fragments of the scoriae 263745 were heated at 1,000° for 30 

 minutes and showed but Httle change; after being held at 1,100° for 

 30 minutes the product was found to be tightly sintered and the glass 

 base of the original material largely crystallized to brown acicular 

 crystals of medium to fairly strong birefringence, refractive index 

 sHghtly above 1.66, and high extinction angles. These optical prop- 

 erties agree with those of pyroxene. The crystals were too small for 

 a more definite identification. Another portion of the scoria, heated 

 to 1,200° for 2 hours and then cooled gradually to 1,050° for one- 

 half hour, where it was held for 2 hours and then cooled to 1,000° 



