246 T. A. BEN DRAT 



scopic analysis of thin sections of samples taken from all levels of 

 these cerros, and a comparison of their composition, texture, and 

 fabric, tend to lend much weight to the assumption that the 

 gneiss throughout belongs to one and the same unit, as it was one 

 and the same type of rock we had to deal with in the granites. 

 This statement is, however, not intended to exclude the possi- 

 bility of local phases representing gradation and variation. 



Like the granite, the gneiss is porphyritic. In a ground-mass 

 commonly of angular or subangular grains of quartz, with rather 

 fringed and dentated outlines, and of feldspars, especially micro- 

 cline and plagioclase, which usually are arranged with their longer 

 diameters parallel or slightly inclined to the plane of rock- cleavage, 

 are imbedded phenocrysts of pegmatite, microcline, albite, labra- 

 dorite, and biotite. All of these minerals, with the exception of 

 the biotite, are more or less allotriomorphic. Microcline seems 

 to take the place of orthoclase to a great extent, wherever the 

 latter is not intergrown with quartz, and we might say that the 

 predominance of microcline and pegmatite constitutes one of the 

 main features of this gneiss. Another feature is the frequent 

 occurrence of crossed lamellae in the plagioclases and a marked 

 tendency to form rather broad lamellae. 



Of the feldspar and quartz the former is in excess. Among 

 the ferro-magnesian minerals, biotite, in phenocrysts of lath-shaped 

 form and in flakes, seems to be almost the only representative. 

 Hornblende is very seldom encountered, though in one instance 

 it was found intergrown with biotite. 



Of accessory minerals an occasional crystal of pyroxene is 

 encountered, while magnetite and hematite are more frequently 

 met with. Garnets are not uncommon and wherever they occur 

 they are idiomorphic. Chlorite is present most probably as a 

 decomposition product from biotite and hornblende. 



As regards inclusions in the different minerals, magnetite 

 occurs in feldspars, arranged more or less parallel to the plane of 

 parting, and in quartz where it occurs as a cloudy matter and in 

 dendrites. Of other inclusions may be mentioned zircon, apatite, 

 titanite, and tourmaline ( ?) in quartz, and very fine glassy particles 

 in the plagioclases. 



