450 Bendrat — Notes on Region about Caicara, Venezuela. 



cline seems to replace the orthoclase to a great extent, wherever 

 the latter is not intergrown with quartz, and it is the predom- 

 inance of microcline, together with that of pegmatite, that con- 

 stitutes one of the main features of this gneiss. Feldspar is 

 more abundant than quartz. 



So far as the ferromagnesian minerals are concerned, hiotite 

 occurs in phenocrysts of lath-shaped form, and in flakes. It 

 seems to be almost the only representative of this group, as 

 hornblende is very seldom encountered in allotriomorphic 

 crystals. 



Of accessory minerals chlorite is present, most probably as a 

 decomposition product from biotite and hornblende ; it is in 

 scaly aggregates. Besides chlorite an occasional crystal of 

 pyroxene is encountered, while magnetite in cubes, though 

 rarely, and dodecahedrons and hematite in flakes, are more 

 frequently met with. Garnets are not uncommon, idiomorphic 

 in form. 



As regards inclusions in the minerals, magnetite may be seen 

 in feldspar, with a more or less parallel arrangement to the 

 plane of parting ; other inclusions are zircon, chlorite, titanite 

 and tourmaline (?) in quartz, actinolite in the feldspars, very 

 fine glassy particles in the plagioclases, apatite in quartz and 

 in pegmatite, the minute needles, or their fragments, being 

 arranged in trains. 



To a greater extent than in the granites we meet in the 

 gneisses with evidence of dynamic metamorphism. The evi- 

 dences of katamorphism, due to stress and shearing, are : the 

 wavy extinction in a number of quartz and feldspar grains ; 

 the bending of lath-shaped biotite; the differential bending 

 and anastomosing of the lamellae of feldspars : cracks and 

 fractures parallel, or normal, to the plane of parting in bio- 

 tite ; the slicing of crystals of pegmatite and microcline ; break- 

 ing of crystals of feldspar and biotite, and displacement of 

 the broken parts by miniature faulting ; granulation zones 

 about grains of pegmatite. 



The gneissoid structure of the rock is also shown in the sec- 

 tions by the direction of the longer axis of a number of grains 

 being parallel, or somewhat inclined, to the plane of schistosity. 

 The Origin of the Gneiss. — The question arises, whether 

 the gneisses are of igneous or of sedimentary origin. 



Evidences in favor of the former are the following : the pres- 

 ence of garnets, which are frequent in the granite of the region ; 

 the regularity and fineness of the foliation, and the apparent 

 uniformity of character of the gneiss both macroscopically and 

 microscopically ; while as negative evidence in favor of it is 

 the absence of minute variations in the thickness of the laminae, 

 as well as the absence of minor plications, of widely dissemi- 

 nated graphite and of any ferruginous beds within the gneiss. 



