704 K. F. MATHER FRONT RANGES OF THE ANDES 



Page 



Limon fault and anticline 753 



Charagua anticline 754 



Oquita-Taimbermi fault 755 



Mandiyuti fault and anticline 757 



Guarui fault 758 



Chireti fault. 758 



Vitiacua fault and anticline 758 



Aguarague fault and anticline 759 



Geologic history 761 



Devonian Period 761 



Permo-Carboniferous and Permo-Triassic time 762 



Late Mesozoic time 762 



Tertiary Period 763 



Quaternary Period 763 



Introduction 



The following notes concerning the general geology of the Front 

 Eanges of the Andes between 18 degrees and 23 degrees south latitude 

 were obtained during a rapid reconnaissance made in January and Feb- 

 ruary, 1920. They are published through the courtesy of Richmond 

 Levering and Company, for whom the geological examinations were made. 

 Very few geological data are available concerning this little-known region, 

 so that it is hoped the material presented here will be of interest and 

 value in spite of the many obvious deficiencies, which in part resulted 

 from the rapidity with which the field observations had to be made. 



S. C. Herold (1920) 2 has recently summarized the scattered literature 

 pertaining to the southern half of the area under discussion. The data 

 which he presents will not be recounted here, as his paper is accessible to 

 all geologists. In March, 1922, while this manuscript was receiving its 

 final revision before publication, my attention was directed by Mr. Chester 

 Washburne to an important memoir which had just appeared from the 

 pen of Dr. Guido Bonarelli (1921). This paper is easily the most impor- 

 tant contribution yet made to a knowledge of the eastern Andes of north- 

 ern Argentina and southern Bolivia. In it the author has gathered to- 

 gether resumes of all earlier reports which deal with the stratigraphy of 

 that and adjacent regions. The most significant of the new data which 

 he presents relate to the age of the so-called "Horizonte Calcareo-dolo- 

 mitico/ 7 previously considered by most authors as belonging to the Cre- 

 taceous system. Dr. Bonarelli presents convincing evidence to prove that 

 this formation is of late Triassic or early Jurassic age. References to 



2 References may be found in the accompanying bibliography. 



