V. PETROGRAPHIC STUDY 



OF THE Specimens of Loess, Tierra Cocida, and Scoria Col- 

 lected BY the Hrdlicka-Willis Expedition 



By Fred. Eugene Wright and Clarence N. Fenneb 



Introduction 



The petrographic examination of the rock specimens collected by 

 the Hrdlicka-Willis expedition in Argentina was undertaken by the 

 Geophysical Laboratory of the Carnegie Institution of Washington, 

 at the request of the Secretary of the Smithsonian Institution. 

 Although descriptive work of this kind is hardly germane to the 

 purposes of the Geophysical Laboratory, it was apparent that, if 

 the problem were to be solved at all, exact laboratory methods of 

 attack would have to be used and that, even in case the material 

 available was not sufficient or characteristic enough for a satisfactory 

 solution, the precise data gathered by the laboratory would never- 

 theless be of value. The descriptions below apply only to the 

 material examined. 



The problem submitted to the laboratory was to determine the 

 kinds and nature of the rocks in the collection. Ordinarily such 

 problems are not difficult and can be solved by the usual petro- 

 graphic methods, but in the present instance several of the rocks 

 are so extraordinary in type that previous investigators have been 

 unable to agree as to their nature, whether volcanic, or sedimentary, 

 or both, and the matter has become one of controversy based more 

 or less on opinion. 



In approaching this problem, three lines of attack were followed: 

 (1) the usual detailed petrographic -microscopic examination of the 

 rocks; (2) chemical study of the different rock types; (3) thermal 

 study of the rocks at different temperatures and comparison of the 

 products thus obtained, with the natural products. From these 

 three lines of independent evidence which will now be presented, 

 it should be possible to obtain a fair idea of the nature of the rocks 

 and possibly also of their genesis. 



Tlie tools and methods used in the examination of the different speci- 

 mens.— OrdmRTily it is not customary to cite the particular instru- 



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