Not only has erosion changed the characteristic slopes of the peaks built of lava-flows and ejected fragments, but in at least one remarkable instance the volcanic energy itself greatly altered the structure it had previously raised. Mount Mazama, situated in southern Oregon in the summit region of the Cascades, is a truncated volcanic cone in the top of which there is an immense depression now partially filled by the waters of Crater Lake (Fig. 21). The main features in the history of this unique mountain with a lake in its summit, as interpreted by Messrs. Dutton and Diller, of the United States Geographical Survey, are as follows: It once stood as a conical peak, similar to several other mountains of volcanic origin in the same region, some 15,000 feet in height; it was then an active volcano with a summit crater filled with lava, but subsequently, for a time at least, became dormant and was occupied by glacial ice. At a later period an escape for the lava was furnished by a fissure or other opening which admitted of a surface discharge at a more or less distant locality, in a manner similar to the escape of the molten rocks from the great volcanoes of the Hawaiian Islands within historic times. This drawing off of the lava from the crater removed the support afforded its walls from within, and the summit portion of the mountain, embracing about three-fourths of its height above the adjacent valleys, fell in and was engulfed. The mountain was thus truncated, and presents the general appearance of similar cones the upper portions of which are known to have been blown away by explosions. But in the case of Mount Mazama, the hypothesis of truncation by explosion seems to be disproved by the absence of the fragments of the portion removed on the slopes remaining or on the surrounding region. After the falling in of the summit of the mountain comparatively mild volcanic explosions followed which built a cone within the great pit or _caldera_ in the summit of the truncated mountain, but without filling it. The space left vacant is now occupied by water, and thus transformed into a lake. The cone built after the catastrophe referred to now forms Wizard Island, near the southwest border of Crater Lake. Some idea of the magnitude of the changes wrought in Mount Mazama by the events recorded in its geology and topography may be obtained from the following facts: Crater Lake has a surface elevation of 6,239 feet above the sea, and is nearly 2,000 feet deep in its deepest part; the precipices surrounding it are from 520 to 1,987 feet high. The whole depth of the depression is therefore 4,000 feet. This _caldera_, as such basins of volcanic origin are termed, is nearly circular, with an east and west diameter measuring 6 miles, and a north and south diameter of 5 miles. The volume of the pit is nearly 12 cubic miles.