76 BtJBEAU OF AMEKtCAN' EtHNOLOGY Ibull. 5'2 



There is nothing, however, in the analysis alone which precludes an 

 igneous origin for this rock. 



No. 263731. Specimen label. "Locality: Miramar. Material: 

 Scoria and Tierra Cocida." 



This specimen includes three distinct t;^'pes of material : A grayish- 

 black scoria, brick-red earthy material, and pale-brown loess. The 

 scoria is vesicular and full of small bubbles. Parts of the outer 

 surface of the scoria are often glassy and smooth and indicative of an 

 original cooling surface. Wliere this surface is fractured, the small 

 cavities are more or less filled with brown earth and with coarser 

 rounded crystals of quartz, plagioclase, magnetite, and pyroxene. 

 These grains have evidently been lodged in their present position 

 either by wind or by water action. On one side of the specimen a brick- 

 red, compact earth fills the small cavities and is e\ddently different 

 from and older than the normal brown loess. In the cavities con- 

 taining the brick-red earth, no rounded, coarser grains of the minerals, 

 noted above, were observed. This red earth is more or less indurated 

 and closely resembles the tierra cocida from Alvear described above 

 and probably owes its color to a baking process at higher tempera- 

 tures. There is, however, no transition between the glassy scoria and 

 this red earth. The glassy walls of the vesicles are sharply marked 

 and the red earth was evidently introduced mto the cavities after 

 their formation and is of later origin. Microscopically the com- 

 position of the scoria and the red earth is noticeabty different. The 

 red earth contains abundant red iron oxide and chalcedony, besides 

 fragments of quartz, sodic plagioclase (oligoclase chiefly) and pjTox- 

 ene and colorless glass. These same minerals are present in the 

 scoriae, but the plagioclase is noticeably more anorthic (andesine and 

 labradorite) and pyroxene is less abundant. The glass is pale-brown 

 and streaky, the colors appearmg often along curved bands as 

 though the molten glass had flowed. The microlites which have 

 crystallized from the glass show medium bkefringence, but are not 

 sufficiently well-developecl to be satisfactorily determined. At 

 many points in the specimen a thin shell of the glass lining the 

 cavities has devitrified and scales off in thin flakes. 



The brown earth is of the usual tj\^e, a friable, pale-brown loess 

 containing argillaceous material and fragments of quartz, plagio- 

 clase (labradorite), pyroxene, magnetite, and glass. This material 

 is obviously later than both the scoria and the red earth. 



Portions of the scoria and of the adjacent earth, each mixed with 

 graphite, were heated simiiltaneously in separate crucibles to 1 ,200° 

 for 2 hours and then cooled slowly for 2 hours from 1,200° to 1,000°. 

 The scoria was found to have melted down to a dark olive-green glass 

 which took the shape of the crucible. Microscopically it contamed 

 remnants of the original mineral fragments and also a few minute 



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