WRIGHT-FEN NBR] PETROGRAPHIC STUDY 89 



loess. Chemically also they are practically identical, as analyses 

 263717 and 263729 show. The percentage of HjO in 263729 is lower 

 than that in 263717 as was to be expected. In both the amount 

 of FeO present is very slight. This is also true of analysis 263722 

 of the tierra cocida from Miramar and is in accord with the condi- 

 tions under which the tierras cocidas have probably been formed. 

 Many of the specimens of tierra cocida are so large and compact 

 that one is forced, in explaining their mode of formation, to assume 

 long-continued and confined heating at a fairly high temperature, 

 such as would be encountered near the contact of an intrusive 

 igneous or volcanic mass, but not beneath an open fire made of grass 

 or small timber. 



(2) Some of the scoriae contain bright-red patches, which resemble 

 the tierras cocidas superficially, but which on closer examination 

 are seen to be glassy and to have been melted, just as the gray parts 

 of the scorige have been melted, the chief difference between the two 

 parts being the state of oxidation of the iron. These Will be con- 

 sidered below, together with the scoriae to which they belong. 



(3) Brown ferruginous earths (specimen 263710) have also been 

 considered tierra cocida by some investigators. A careful micro- 

 scopic examination of these specimens has shown that they are 

 simply loess in which ferruginous material abounds (ferruginous 

 concretion) and acts as a weak cementing material, causing the 

 specimens to appear more indurated than the surrounding loess. 

 On immersing such specimens in water they are seen to crumble 

 and break down readily. The red tierras cocidas or baked earths 

 do not crumble in the least under these conditions. 



THE SCORLE 



Microscopic petrograpMc features. — Under the microscope the 

 scoriae from both Miramar and Necochea exhibit abnormal features. 

 Irregular fragments of quartz, plagioclase, pyroxene, and mag- 

 netite occur embedded in a colorless-to-pale-brown glass, out of which 

 aggregates of acicular crystals, probably p3n'oxene, crystallize. The 

 mineral fragments have been considered phenocrysts by some 

 observers. On this assumption the rock would be classed with the 

 andesites and has been described as such. The chemical analyses 

 of the present paper might be considered confirmatory evidence in 

 favor of this view. A careful microscopic examination of the mineral 

 fragments, however, precludes this hypothesis. The mineral frag- 

 ments are broken and irregular in outline. (PL 7, a and pi. 8, a.) They 

 were evidently not stable in the molten glass and were attacked and 

 dissolved in part by the same. The plagioclase feldspars are not 

 uniform in composition. In the same microscope-field fragments 



