248 T. A. BEN DRAT 



of fine-grained amphibolitic gneiss, varying in thickness from two 

 to three feet, strikes N.-S. A few feet below the top of Cerro de 

 los Spiritos a felsite vein about one foot thick extends E.-W. and 

 stands two feet above the surrounding coarse-grained granite, so 

 that from a distance it has the appearance of the remnant of a 

 brick wall. A microscopic investigation showed that its main 

 constituents are quartz and plagioclase in nearly equal propor- 

 tions, while some dark-brown and yellow biotite occurs as an 

 accessory mineral. A microscopical examination of the N.-S. 

 quartz veins shows that they are made up of very small grains of 

 quartz with dentated margins in most cases and most intricately 

 interlocked; of bronze-brown to dark-green amphibole in irregular 

 patches; occasional tourmaline in radiate aggregates; some garnets, 

 and some magnetite. 



In concluding, the writer wishes to remark that it would be 

 of great interest to ascertain the geologic and petrographic nature 

 of other outliers in those waste plains that approach the Orinoco 

 from the S. and from the E., as well as the character of the rocks 

 that make up the bulk of the Guiana mountain system, in order 

 to bring out facts that would bear on the relations of those cerros 

 to this, most probably, Archaean center. 



