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PETROGRAPHIC STUDY 85 



closely the natural tierras cocidas 263721, 263729, 263731, 263732. 

 Whether the latter were produced by the burning of grass can not be 

 definitel}^ ascertained. Their relatively compact texture and the 

 absence of grass-leaf impressions are not in favor of this view. 



These specimens do not disintegrate when immersed in water. 

 The water solution does show, however, the presence of a fair amount 

 of sodium chloride. 



No. 263739. Fragments of this artificial tierra cocida were held at 

 1,100° for 2J hours and were found to ha've melted to a viscous glass, 

 in which many of the original mineral fragments are still visible 

 under the microscope. The glass is so filled with fme particles of red 

 iron oxide that it is opaque at many points. 



It is of interest to note the lack of compactness of this earth baked 

 by the burning of grass. The product is extremely porous and full 

 of holes, many of which evidently mark the position of former grass 

 roots. The specimens of tierra cocida from Miramar and Sala- 

 dillo, on the other hand, are large, compact masses; for their forma- 

 tion a much greater quantity of uniform, confined heat was required 

 than for the Rio Colorado material. The source of heat supply for 

 the baking of the tierra cocida was probably volcanic contact 

 action. This hypotliesis is developed at greater length below in con- 

 nection with the formation of the scoriae, but it applies with almost 

 equal force to the formation of the tierra cocida of this region. 

 The assumption that the large specimens of tierra cocida were 

 formed simply by the action of open fires is hardly possible in view 

 of the quantity of heat involved, which must have acted through a 

 period of time on large masses of material to have produced the 

 effects observed. 



SPECIMEN FROM SAN BLAS 



No. 263744. Specimen label. ''Locality: Coast near San Bias, 

 north of Rio Negro. Material: Scoria or j^umice." 



A dark, reddish-brown, pumiceous scoria containing here and there 

 a minute feldspar phenocryst, but otherwise aphanitic and stony in 

 appearance. Under the microscope occasional phenocrysts of plagio- 

 clase (labradorite, AbiAnJ and pyroxene were observed. The 

 groundmass is filled with fine specks of opaque, ferruginous substance 

 which renders it for the most part nontransparent. Fine laths of 

 plagioclase were noted in the clearer parts of the groundmass, but 

 so far as could be determined it is largely isotropic and glassy. Both 

 megascopically and microscopically this specimen does not differ in 

 any essential feature from many andesites, and there is no reason to 

 doubt its volcanic origin. 



