244 T. A. BEN DRAT 



finally, the sabana itself. They lodge between the rocks of the 

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

 in interspaces. The loam becomes very pliable and soft during 

 the rainy season, so much so as to allow mud turtles to plow 

 furrows from half a foot to a foot deep in it. Local torrents some- 

 times carve channels down to the underlying laterite. The walls 

 of these channels become extremely indurated during the dry 

 season, rendering traveling across the sabana under such conditions 

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



PETROGRAPHY OF THE GRANITES AND GNEISSES THAT CON- 

 STITUTE THE CERROS IN THE SABANA ABOUT CAICARA 



I. THE GRANITES 



The granites of the region under consideration comprise those 

 of the Cerro de Cabruta, the Cerro de los Spiritos, and those at 

 the base of the Cerro de Arinoza. A detailed microscopical study 

 of specimens taken from different levels shows that in all three 

 cerros the type of granite is essentially the same. This statement, 

 however, has to be modified in so far that, at one end of this ellip- 

 tic area, the granite is decidedly leukocratic while at the other it 

 is melanocratic. While the Cerro de Cabruta consists of a more 

 or less pronounced quartzose granite in its upper levels and its 

 top, the Cerro de los Spiritos reveals a decided predominance of 

 hornblende in the granite of its top. 



Though described here in general terms as granite, this rock 

 is more correctly designated as quartz monzonite porphyry. Its 

 ground mass is made up of more or less angular or subangular 

 crystals of quartz and feldspar through which the leading miner- 

 als are sprinkled as phenocrysts. These phenocrysts are quartz, 

 which sometimes predominates over the feldspars, occasional 

 orthoclase associated with, or replaced by, microcline, the soda- 

 lime and lime-soda feldspars albite and labradorite, and finally 

 biotite, associated with or entirely replaced by hornblende. 



Quite a number of quartz and feldspar phenocrysts show enlarge- 

 ment by secondary growth as revealed by zonal extinction or 

 by a more or less faint ring of dark material, apparently repre- 

 senting a coating of the original grain. 



