Had the glaciers that once covered the range been melted at once, leaving the entire surface bare from top to bottom simultaneously, then of course all the lakes would have come into existence at the same time, and the highest, other circumstances being equal, would, as we have seen, be the first to vanish.  But because they melted gradually from the foot of the range upward, the lower lakes were the first to see the light and the first to be obliterated.  Therefore, instead of finding the lakes of the present day at the foot of the range, we find them at the top.  Most of the lower lakes vanished thousands of years before those now brightening the alpine landscapes were born.  And in general, owing to the deliberation of the upward retreat of the glaciers, the lowest of the existing lakes are also the oldest, a gradual transition being apparent throughout the entire belt, from the older, forested, meadow-rimmed and contracted forms all the way up to those that are new born, lying bare and meadowless among the highest peaks.

[Illustration:  THE DEATH OF A LAKE.]

A few small lakes unfortunately situated are extinguished suddenly by a single swoop of an avalanche, carrying down immense numbers of trees, together with the soil they were growing upon.  Others are obliterated by land-slips, earthquake taluses, etc., but these lake-deaths compared with those resulting from the deliberate and incessant deposition of sediments, may be termed accidental.  Their fate is like that of trees struck by lightning.

The lake-line is of course still rising, its present elevation being about 8000 feet above sea-level; somewhat higher than this toward the southern extremity of the range, lower toward the northern, on account of the difference in time of the withdrawal of the glaciers, due to difference in climate.  Specimens occur here and there considerably below this limit, in basins specially protected from inwashing detritus, or exceptional in size.  These, however, are not sufficiently numerous to make any marked irregularity in the line.  The highest I have yet found lies at an elevation of about 12,000 feet, in a glacier womb, at the foot of one of the highest of the summit peaks, a few miles to the north of Mount Hitter.  The basins of perhaps twenty-five or thirty are still in process of formation beneath the few lingering glaciers, but by the time they are born, an equal or greater number will probably have died.  Since the beginning of the close of the ice-period the whole number in the range has perhaps never been greater than at present.

A rough approximation to the average duration of these mountain lakes may be made from data already suggested, but I cannot stop here to present the subject in detail.  I must also forego, in the mean time, the pleasure of a full discussion of the interesting question of lake-basin formation, for which fine, clear, demonstrative material abounds in these mountains.  In addition to what has been already given on the subject, I will only make this one statement.  Every lake in the Sierra is a glacier lake.  Their basins were not merely remodeled and scoured out by this mighty agent, but in the first place were eroded from the solid.