these are so considerably obscured by post-glacial erosion, and by a growth of forests, underbrush, and weeds, that only the patient and educated eye will be able to recognize them beneath so many veils.

The ice-sheet of the glacial period, like an immense sponge, wiped the Sierra bare of all pre-glacial surface inscriptions, and wrote its own history upon the ample page. We may read the letter-pages of friends when written over and over, if we are intimately acquainted with their handwriting, and under the same conditions we may read Nature's writings on the stone pages of the mountains. Glacial history upon the summit of the Sierra page is clear, and the farther we descend, the more we find its inscriptions crossed and recrossed with the records of other agents. Dews have dimmed it, torrents have scrawled it here and there, and the earthquake and avalanche have covered and erased many a delicate line. Groves and meadows, forests and fields, darken and confuse its more enduring characters along the bottom, until only the laborious student can decipher even the most emphasized passages of the original manuscript.

METHODS OF GLACIAL DENUDATION

All geologists recognize the fact that glaciers wear away the rocks over which they move, but great vagueness prevails as to the size of the fragments, their abundance, and the way in which the glacial energy expends itself in detaching and carrying them away. And, if possible, still greater vagueness prevails as to the forms of the rocks and valleys resulting from erosion. This is not to be wondered at when we consider how recently glacial history has been studied, and how profound the silence and darkness under which glaciers prosecute their works.

In this article I can do little more for my readers than indicate methods of study, and results which may be obtained by those who desire to study the phenomena for themselves. In the first place, we may go to the glaciers themselves and learn what we can of their weight, motions, and general activities*___

Here I would refer my readers to the excellent elementary works of Agassiz, Tyndall and Forbes.

gion- namely, these junipers. Barring extraordinary accidents, the curve of their growth should be essentially like that of the sequoia, having the same time-scale, and differing only in the scale of magnitude—that is, the ordinates of the juniper curve should at all points be proportional to those of the sequoia curve. The problem is therefore to find the constant ratio between the two.

Turning now to the first juniper of the above list, we see that the initial point of its curve must of course be at zero of the century scale. A second point is also known, determined by its age of 247 years (at P) and its diameter of 24 inches (Pa). By continuing the vertical coördinate Pa to the sequoia curve at á, we get the ordinate of that curve at the 247-year point Pá, -that is, the radius-measurement of the sequoia at that age, 29.5 or 0.8 is therefore the ratio sought. Applying this ratio in succession to each of the 200-year measurements of the sequoia growth, we shall have the corresponding measurements of our juniper according to our forecast, which, when plotted, give us curve No. 2 of the chart. In like manner the age and measurements of the Glen Alpine tree result in curve No. 3 of the chart.

The scheme assumes that by the time such a tree as these has reached the age, say of 250 years, it has struck its true pace— has found its proper scale of growth. Forecasting on this basis the "expectation of growth" for these two trees, we find that the mountain juniper might attain the five feet of diameter assigned to its class at about the age of 1300 years, and the valley juniper the seven feet assigned to its class at about 1500. The forecast is probably a little too favorable for the junipers

1

2

3

which occupy exposed positions on the mountain ridges. For the sequoia-record which serves as the basis of the forecast is that of a tree uncommonly well defended from the accidents and stresses which sap the strength and check the growth of middle and later life. Serious damage by fire it seems to have escaped altogether. The deadly freezing and drying winds of winter which the junipers must face singly as they stand scattered about on the storm-beaten heights, could not harm this sequoia deep in its narrow dell and girt about by its giant brethren. So far then as this consideration has weight, it points to a date still later than that just now named for the attainment of its supposed maximum size.

There is also another consideration which seems to point in the same direction. The largest junipers that I have chanced upon have always been found far up on the mountain flanks. Their curve of growth therefore should be represented not by curve No. 2, but by the more pinched and starved No. 3. I feel sure that I have seen among them trees of more than seven feet in diameter, but never having had the wit to measure them, I cannot insist upon that.-Let their maximum be seven feet in diameter. According to curve No. 3 how old should they be? One actually hesitates to name the figure.

On the other hand, the enormous age which used to be claimed for the giant sequoias has been steadily cut down by the increase of definite knowledge, until now it appears that the greatest age demonstrated by actual counts is no more than 2200 or 2300 years. It would seem then that the juniper is actually in the race of life alongside of its big brother the sequoia !

May 16, 1917

how they detach, transport, and accumulate rocks from various sources. Secondly, we may follow in the tracks of the ancient glaciers, and study their denuding power from the forms of their channels, and from the fragments composing the moraines, and the condition of the surfaces from which they were derived, and whether these fragments were rubbed off, split off, or broken off.

The waters which rush out from beneath all glaciers are turbid, and if we follow them to their resting-places in pools we shall find them depositing fine mud, which, when rubbed between the thumb and finger, is smooth as flour. This mud is ground off from the bed of the glacier by a smooth, slipping motion accompanied with immense pressure, giving rise to the polished surfaces we have already noticed. These mud particles are the smallest chips which glaciers make in the degradation of mountains.

Toward the end of the summer, when the winter snows are melted, particles of dust and sand are seen scattered over the surfaces of the Sierra glaciers in considerable quantities, together with angular masses of rock derived from the shattered storm-beaten cliffs that tower above their heads. The separation of these masses, which vary greatly in size, is due only in part to the action of the glacier, although they all are borne down like drift on the surface of a river and deposited together in moraines. The winds scatter down most of the sand and dust. Some of the larger fragments are set free by the action of frost, rains, and general weathering agencies; while considerable quantities are borne down in avalanches of snow, and hurled down by the shocks of earthquakes. Yet the glacier performs an important part in the production of these superficial effects, by undermining the cliffs whence the fragments fall. During my Sierra explorations in the summers of 1872 and 1873, almost every glacier I visited offered illustrations of the special action of earthquakes in this connection, the earthquake of March, 1872, having just finished shaking the region with considerable violence, leaving the rocks which it hurled upon the ice fresh and nearly unchanged in position.

But in all moraines we find stones, which, from their shape and composition, and the finish of their surfaces, we know

were not thus derived from the summit peaks overtopping the glaciers, but from the rocks past which and over which they flowed. I have seen the north Mount Ritter Glacier and many of the glaciers of Alaska in the act of grinding the side of their channels, and breaking off fragments and rounding their angles by crushing and rolling them between the wall and ice. In all the pathways of the ancient glaciers, also, there remain noble illustrations of the power of ice, not only in wearing away the sides of their channels in the form of mud, but in breaking them up into huge blocks. Explorers into the upper portion of the middle granite region will frequently come upon blocks of great size and regularity of form, possessing some character of color or composition which enables them to follow back on their trail and discover the rock or mountain - side from which they were torn. The size of the blocks, their abundance along the line of dispersal, and the probable rate of motion of the glacier which quarried and transported them, form data by which some approximation to the rate of this sort of denudation may be