Bendrat — Notes on Region about Caicara, Venezuela. 449 



orthoclase (not so much over the microcline), and is chiefly 

 labradorite. 



Biotite occurs in lath-shaped crystals, shreds and flakes : 

 amphibole mostly in prismatic forms. 



An intergrowth of biotite and amphibole was observed, the 

 former in the latter, demonstrating conditions in the magma 

 that favored the contemporaneous development of these min- 

 erals. Apatite and microlites of titanite occur as inclusions in 

 the biotite, while the amphibole carries needles of apatite and 

 grains of magnetite. Biotite has been observed in feldspar, 

 and also apatite needles, while the quartz has dendrites of mag- 

 netite, also apatite and occasionally zircon. Liquid inclusions 

 are sometimes present in the quartz. Titanite also occurs in 

 free crystals. 



Secondary minerals are calcite, which might have been 

 derived from the feldspars, and chlorite, from the biotite, and 

 last but not least, garnet, more or less idiom orphic. 



Wavy extinction in quartz and feldspar phenoerysts and 

 microscopic grains ; bending and breaking, as well as slicing, 

 of feldspars and biotite ; granulation in feldspar and quartz, 

 and complete crushing of titanite crystals strongly suggest 

 repeated katamorphism brought about by dynamic stress and 

 shearing. 



The occurrence of more or less advanced decomposition and 

 decoloration of considerable part of the biotite indicates the 

 agency of descending waters, while the secondary growth of 

 minerals, involving zonal structure, and the formation of vein- 

 lets, where the crystals were broken, tell rather of ascending 

 solutions. 



The Gneisses. — The gneisses of the cerros in the sabana of 

 Caicara comprise those of the Cerro de Caicara, Cerro de Ari- 

 noza with the exception of its base, and those of the Cerro de 

 Morano. 



The study of thin sections prepared from specimens taken 

 from the foot, the top and sides of these cerros shows by the 

 comparison of their composition, structure, texture, and fabric 

 that they are essentially one and the same gneiss, as it was 

 one type in the granites. This is, however, not intended to 

 exclude local phases that represent gradation and variation. 

 The petrographic description of these gneisses is as follows : 

 In a groundmass of angular to subangular grains of quartz, 

 with rather fringed and dentated outlines, and of microcline 

 and plagioclase, arranged with their larger diameter either par- 

 allel or more or less inclined to the plane of rock cleavage, are 

 imbedded phenoerysts of pegmatite, microcline, and plagioclase, 

 as well as biotite. All these minerals, with the exception of 

 the biotite, exhibit more or less allotriomorphic forms. Micro- 



