The Vishnu Schist Formation
The oldest rocks within the Grand Canyon are exposed within Granite Gorge aria are characteristically dark somber gray. They respond to erosion to form a steep-walled V-shaped gorge (Text-fig. 56) through which the Colorado River flows from Mile 77 downstream to beyond Phantom Ranch, These (lark colored rocks are evidence of extensive deformation, during which they were subjected to intense heat and pressure and the effects of fluids and gases. The original sedimentary or volcanic characters have been extensively modified and in some cases obliterated. Early Precambrian rocks are not stratified but possess a planar structure known as foliation, resulting from reorientation of platy minerals, crystals, and grains in response to deformation. Foliation throughout most of Granite Gorge is nearly vertical which contrasts with the horizontal stratification of the overlying younger rocks.
Three major rock bodies are found within the Early Precambrian complex. The first encountered on the river trip consists of metamorphosed sedimentary rocks in which some relict. sedimentary structures are preserved. This body of baked and altered rocks is known as the Vishnu Schist and is exposed downstream beyond Hance Rapids to near Zoroaster Canyon. They represent part of the older rocks of the earth's crust. Very little detailed information can be gained about their environment of deposition since the original character of the rock has been nearly completely obliterated.
Downstream from Zoroaster Canyon is a sequence of metamorphic rocks which differ in composition, color, and texture from the Vishnu Schist but superficially appear similar to it because of their degree of metamorphism, These rocks are known as the Brahma Schist and probably represent metamorphosed volcanic rocks. Numerous granitic dikes have intruded into both the Vishnu and Brahma schists. Most of these dikes are nearly vertical and parallel to foliation of the schists and stand out in marked contrast to the metamorphic material. Rocks of the dikes are characteristically pink, light colored, and composed of large interlocking crystals of feldspar and quartz, many of which are over a foot in diameter. These intrusions become very numerous in many areas and in some localities make up more than 50 percent of the rock body. Near Zoroaster Canyon dikes are particularly common and one large massive intrusion is dissected by the canyon.
Large granite bodies of the Inner Gorge are referred to as the Zoroaster Granite and represent a period of igneous activity after deposition and metamorphism of both the Brahma and Vishnu Schists but before deposition of the overlying Grand Canyon Series and Paleozoic formations.
'The Vishnu Schist in the upper part of the gorge contains many pink pegmatite dikes. Many units within the schist are well foliated and may appear similar to a gneiss. Locally, relict bedding can be seen indicating a sedimentary origin. Foliation is nearly vertical. The gray-green walls of the Vishnu Schist are crisscrossed by dikes of granite. The ragged, ledgy, V-shaped character of the inner gorge is evidenced downstream.
Geology Studies Vol. 15- Part 5-1968