44S Bendrat — Notes on Region about Caicara, Venezuela. 



upper portions of the islands, the upper terraces at an eleva- 

 tion of about fifty feet above the average level of the lower 

 terraces, and the sabana itself. They lie between the rocks of 

 the granites and gneisses, wherever these come to the surface, 

 filling in interspaces and fissures. The loam of the sabana 

 becomes exceedingly plastic and soft during the rainy season, 

 allowing turtles to dig furrows and ditches of from half a foot 

 to one foot deep into it, and the torrents to carve deep 

 channels, sometimes down to the underlying laterite. The 

 walls of these ditches and channels become extremely indu- 

 rated during the dry season, thus rendering traveling across 

 the sabana under such conditions exceedingly difficult and tire- 

 some, especially where there are no roads or paths available. 



Petrography. 



In describing the granites and gneisses of the cerros petro- 

 graphically, the writer will describe them in a sequence that is 

 determined by their genetic character, thus taking up the 

 granites first and then the gneisses. 



The Granites. — The granites of the region are those of the 

 Cerros de Cabruta and de los Spiritos and those that form the 

 base of the Cerro de Arinoza. 



The microscopical study of thin sections from different 

 places shows that there is essentially the same type of granite 

 in all three cerros. This statement, however, has to be modi- 

 fied in so far that at the two ends of this seemingly elliptic 

 area, the granite shows a tendency to acidity at one and basid- 

 ity at the other. For, while the Cerro de Cabruta consists of a 

 granite rich in quartz in its upper levels and its top, the Cerro 

 de los Spiritos has a predominance of the hornblende in the 

 rock of its top. 



The petrographic features which characterize this granite 

 are as follows : 



In a mass of angular or subangular grains of quartz and 

 feldspar occur phenocrysts of the leading minerals, viz., the 

 quartz orthoclase which is occasionally replaced by microcline 

 as well as soda-lime feldspars, and lastly biotite, which is 

 associated with hornblende, the latter sometimes replacing it 

 entirely. 



A number of quartz and feldspar phenocrysts show en- 

 largement by secondary growth, revealed by a faint ring of 

 dark material, or by zonal extinction. 



The frequent occurrence of micro-pegmatitic texture suggests 

 conditions in the eutectic magma favoring simultaneous crystal- 

 lization of quartz and feldspar. 



Of the feldspathic minerals the soda-lime group prevails over 



