WEIGHT-FENNER] PETROGRAPHIC STUDY 97 



As a natural result of its brecciated character the structure is most 

 varied, which is the more pronounced between crossed nicols. It is 

 partly spherulitic and axiolitic and partly cryptocrystalline and in 

 places it is microcrystalline in irregular grains." 



In many respects this description might serve for the thin sections 

 of the Miramar and Necochea scoriae. Similar irregular broken 

 fragments of quartz, plagioclase, and other minerals are present and 

 the groundmass is variable both in composition and in degree of 

 crystalUzation. The hypothesis suggested above for the genesis of 

 the scorise of Necochea and Miramar is therefore not new to geology 

 and is without question a probable working hypothesis for rocks of a 

 similar nature in any other part of the world. 



Petrographic-Microscopic Examination of Bones Collected 

 BY Doctor Hrdlicka 



The writers received in addition to the above collection of rocks 

 a small collection of bones with the request that their mineral com- 

 position be determined so far as possible. The specimens were 

 examined accordingly in powder form and no attention was given to 

 the structural features of the bones. In many of the bones fine 

 mineral particles from the adjacent loess, in which they probably 

 occurred, were observed but, as these minerals bear no relation to the 

 alteration of the bones, their presence is not recorded in the descrip- 

 tions below. 



Practically only one type of alteration was observed. The cryp- 

 tocrystalline bone substance is replaced by a mixture of micro-crys- 

 talline calcite and of a weakly- to-medium birefracting substance of 

 refractive index about 1.61 and resembling in appearance the mineral 

 described under specimen 263738 above. In some of the specimens 

 the calcite is not present and the weakly birefracting substance is 

 proportionately more abundant. In all of the specimens a consider- 

 able amount of fine, low-refracting, often isotropic or cryptocrystalline, 

 material occurs but is too fine for satisfactory determination. In 

 the following paragraphs the results of the examinations of the 

 powdered specimens of bone are recorded briefly. 



The conclusions which may be indicated by these observations 

 have probably been given in the foregoing Hrdlicka-Willis report, 

 which the present writers have not seen. 



No. 263748. Specimen label. " 'Fossil' man. Human hones 

 from the Necochea slceleton." 



In the powder of this bone fine, microcrystalline calcite is abundant. 

 The calcite is coarser-grained in this specimen than in any of the 

 other bones in the collection. Grains of a weakly-to-medium bire- 

 fracting substance, usually pale-yellow in transmitted light and of 

 21535°— Bull. 52—12 7 



