A lofty mountain range still more complex in its history is the Sierra Nevada of California. First, thick strata were highly folded, upraised, and intruded by great masses of molten granite. Erosion then proceeded to cut the range down to hills, after which a great fracture (fault) developed along the eastern side and the Sierra Nevada earth block was notably tilted with steep eastern front and long western slope. Erosion has considerably modified the eastern fault face, and the deep canyons like Yosemite, King's River and American River, have been carved out of the western slope of the great tilted fault block. Geologically recently the central to northern portion of the range has been affected by volcanic action, streams of lava in some cases having flowed down the valleys. CHAPTER XI A STUDY OF LAKES Lakes are ephemeral features on the face of the earth. Compared to the tens of millions of years of known earth history, lakes, even large ones, are very short lived. They may, in truth, be regarded as merely results of the temporary obstructions to drainage. Lake basins are known to originate in many ways, and there are various means by which they are destroyed. Not attempting an exhaustive, scientific treatment of the subject, our present purpose may be well served by describing and explaining some of the better known and more remarkable lakes of the world. Even a cursory examination of a large map of the world reveals the fact that the regions of most numerous lakes are those which were recently occupied by glaciers--either the vast ice sheets of the Glacial epoch or mountain (or valley) glaciers. This is because more lakes of the present time have come into existence as direct or indirect results of glaciation than by any other cause. A considerable number of these lakes occupy rock basins which have been eroded or excavated by the direct action of flowing ice. Small lakes of this sort are commonly found in the upper parts of valleys formerly occupied by mountain or so-called Alpine glaciers, because there the excavating power of such glaciers was especially effective. More rarely rock basins have been scoured out by glaciers farther down their valleys. Many lakes occupy rock basins excavated by ice in the high Sierra Nevada and Cascade Ranges, in the Rocky Mountains from Colorado into Canada, in the Alps, and in the mountains of Norway. Few, if any of them are, however, large or famous. Other lakes, some of very considerable size, occupy rock basins scoured out by the passage of the great ice sheets of the Glacial epoch in North America and Europe, though they are less common than formerly supposed. Some of the many lake basins of Ontario, Canada, are quite certainly of this origin, as might well be expected, because the power of the great ice sheet was there in general notably greater than south of the Great Lakes where the tendency was to unload or deposit the eroded materials as shown by the great accumulations of