I have no doubt of the truth of this theory of formation as it relates to the Sierra Nevada ranges as they exist to-day, for the intrusion of the granite into the slate formations suggests a force far greater than can be ascribed to volcanic action alone. The _previous_ condition of the “continental mass” can not be so well imagined; yet reasoning from what we know of the present condition of the Sierras we may with propriety assume that great changes had occurred in the territory embracing the Sierras Nevada long prior to their upheaval. The changes that have occurred since are too abundant and enduring to require more than a reference to the localities. The “glacier pavements” of the Sierras are so conspicuous that, as Mr. John Muir says: “Even dogs and horses gaze wonderingly at the strange brightness of the ground, and smell it, and place their feet cautiously upon it, as if afraid of falling or sinking.” These glacier-smoothed rocks “are simply flat or gently undulating areas of solid granite which present the unchanged surface upon which the ancient glaciers flowed, and are found in the most perfect condition in the sub-alpine region, at an elevation of from 8,000 to 9,000 feet. Some are miles in extent, only interrupted by spots that have given way to the weather, while the best preserved portions are bright and stainless as the sky, reflecting the sunbeams like glass, and shining as if polished every day, notwithstanding they have been exposed to corroding rains, dew, frost and snow for thousands of years.” This statement of Mr. Muir will especially apply to the “glistening rocks” at the sources of the Merced and Tuolumne rivers, in view on this trail through the Mono Pass. The evidences of past glacial action in polishing the domes, mountains and valleys _above_ the Yosemite valley, are too undeniable for controversy, but how much of the Yosemite itself may have been produced by glacial action will probably always remain a theme for discussion among geologists. Prof. Samuel Kneeland, the well known author of “Wonders of the Yosemite,” in a letter to me upon the subject, says: “I think there can be no doubt that the valley was filled, and 1,000 feet above, by ice--that while the _mass above_, moved, that in the valley, conforming to its configuration, was comparatively stationary, lasting much longer than the first, gradually melting to a lake, now represented by the Merced river.