Granites derive their name from their granular texture. They are formed by the slow cooling of molten masses that have been forced into older rocks from the earth’s interior. From a similar source are formed lavas and volcanic ash, but these flow out or are ejected on the surface of the earth where they cool so rapidly that no crystals form. Exceptionally fast cooling or chilling of molten masses, moreover, forms volcanic glass or obsidian. It is by the application of this same principle that crystal forming is prevented in the manufacture of common glass.
The large size of the crystals forming the granite that fills cracks and fissures of the Inner Gorge at Grand Canyon indicates the considerable depth at which it was formed and is further evidence of the great mountains that existed in this region during the first era in geologic history.
CHAPTER II
THE ALGONKIAN ERA
TILTED ROCKS OF THE SECOND ERA. GRAND CANYON
Rocks formed during the second great era of the earth’s history are distinctive in several respects. They are not highly altered or completely changed in form and structure as are those of the oldest era, but are largely free from such changes and, for the most part, similar to rocks which are seen in the process of formation today. Furthermore, they are known to contain definite traces of plant life, though no certain forms of animal life have yet been found in them. They represent a period probably as great as all of subsequent time.
ALGONKIAN ROCKS IN GRAND CANYON
Along Bright Angel Canyon and in several other places in the Grand Canyon, rocks of Algonkian age, representing accumulations of sediments several thousand feet in thickness, are found. Below and to the north of Desert View (southeast of Cape Royal on the North Rim) they form the open floor of the Canyon. Everywhere the most conspicuous layer of this series is a mud rock of brilliant vermilion color. However, the rocks also include a conglomerate or pebble layer, a dark limestone formed principally by plants, and a purple quartzite made by the consolidation of the grains of a sandstone.
FORMATION OF MOUNTAINS
(THE ALGONKIAN ERA)
The Algonkian rocks of the Grand Canyon region were bent and broken into mountains at an early date. In many places sloping layers showing the steep angle at which they were tilted are easily visible, even from the Canyon rim. Folded areas and strata which have been shattered are also conspicuous features here and there. The mountains which they formed, however, are now missing for they were worn away in large measure by slow erosion. Today only remnants—small hills on a general level surface—remain in the lower parts of Grand Canyon to tell the story.