Bendrat — Notes on Region about Caicara, Venezuela. 443 



Art. XXXVII. — Geologic and Petrographic Notes on the 

 Region about Caicara, Venezuela / by T. A. Bendrat, 

 Turners Falls, Mass.* 



In the winter of 1908-09 the writer made some geographic 

 and geologic studies in the interior of Venezuela, his field of 

 investigation being the region immediately west of the El 

 Caura district, at the famous bend of the Orinoco, an area of 

 about 1500 square kilometers, that was hitherto very little 

 known, being mapped. 



While the general results of this survey have been summed 

 up elsewhere,f it is to the geologic and especially the petro- 

 graphic features of the region that this paper relates. 



General Geology. 



The study of the geology of the region shows an underlying 

 bedrock formation of gneisses and granites and on this the 

 Sabana deposits. 



The Gneiss and Granite Basement. 



The bed rock consists of a series of granites and gneisses 

 which, wherever they come to the surface, show a prevalence 

 of gneiss over the granite. They rise from the bottom of the 

 Orinoco channel, constituting the foundation of many of the 

 islands ; they are exposed in the banks of the river ; they 

 appear as cliffs above the water-level during the dry season, 

 they also form the bulk of the so-called Cerros' hills and ridges 

 rising above the plain of the " Sabana," and increasing in 

 height in proportion to their distance from the Orinoco, and 

 which may be regarded as the outliers of the Guyana mountain 

 system in the south. These cerros are portions of the complex 

 of granites and gneisses which have resisted the process of 

 erosion partly because of a number of veins and dikes which 

 traverse them in various directions, but mainly ISL-S. E.-~W\, 

 and N.W.-S.E. 



The Isla de Maria Luisa opposite Cabruta was not visited, 

 but examination of the shores of Isla de Caicara, opposite the 

 village of Caicara, showed that its bed-rock is a purple weather- 

 ing, drab-colored granite with N.-S. and E.-W. cleavage. 



* The writer desires to express in this place his high obligations to Prof. 

 B. K. Emerson of Amherst College, who was kind enough to have the petro- 

 graphic microscopes at Smith College, Northampton, Mass., placed at his dis- 

 posal. 



f Petermann's Geographische Mitteilungen , lvi, No. 5, 1910. Geographen 

 Kalender, 1909, p. 221. 



