LE PANACHE BLANC

HILDA LAURA NORMAN

The sky is scattered with long white plumes.
I wonder what heroes left them so?
Maybe the wind, or maybe the ghosts
Of Henry the Fourth and Cyrano.

Slowly the plumes begin to move,
Feathery light with the winds that blow,
Followed by soldiers of times gone past
Of Henry the Fourth and Cyrano.

Off they ride in the spreading blue
As they used to ride in the long ago,
Off to brave deeds behind the plumes
Of Henry the Fourth and Cyrano.

THE SIERRA NEVADA

ANDREW C. LAWSON

I

In the popular mind the Sierra Nevada is thought of as an aggregate. The term is not plural but the range is referred to as the mountains. The usage constrains the geologist to the same plural reference, but this is only an illustration of a rather common influence of popular usage upon scientific thought, for to him the Sierra Nevada constitutes a magnificent unit, one of the finest examples on the face of the globe of a single range, the type of its class. In order to appreciate this comprehensive point of view, we must begin by a discrimination of the forces of nature, which are engaged in the evolution of the geomorphic features of mountains in general, into two classes. These are on the one hand the formative or constructive forces, and on the other the degradational or destructive forces. All mountains are due primarily to the formative forces of upheaval or of accumulation or of both. Mountains of accumulation are such as have been built up by volcanic eruption about a vent or vents in the earth's crust, as in the case of Mt. Shasta or the volcanic range of Salvador and Guatemala. The Andes of South America owe their lofty character to the coöperation of the forces both of upheaval and of accumulation. For while the high peaks

of the Andes are volcanic piles, yet these are sessile upon the summits of mountains due wholly to the upheaval of the earth's crust. Similarly, the Caucasus enjoys its reputation as the loftiest range in Europe from the fact that it consists of a volcanic pile, 8000 feet in height, perched upon the summit of a mountain 10,000 feet high due entirely to elevatory forces. Mountains which have originated solely by upheaval have not all the same general structure. The elevatory process has operated differently in different cases. The Black Hills of South Dakota constitute a mountain mass due to the simple doming of the earth's crust locally. The great sheets or strata of sedimentary rock, which underlie the surrounding plains in practically flat attitude, have there been so elevated that they dip away in all directions from the center of upheaval toward the unaffected region. The Uinta range is, in its essential features, a broad low arch, flanked on the sides by planes of rupture, as if the upward bulging of the earth's crust had exceeded the accommodation afforded by bending, and could only be further effected by dislocation and the bodily upward thrust of the long arched block lying between the flanking ruptures. In other mountains the single arch or dome is replaced by repeated folds, troughs, or synclines alternating with arches or anticlines, the axes of the folds being more or less parallel. In such mountains the folds may be fairly symmetrical, as in the case of the Jura, or they may be distinctly asymmetric, as in the case of the Appalachian Mountains. In either case we can not, on reflection, escape the conclusion that the upheaval which gave birth to the mountains was due to excessive lateral compression in the earth's crust and a consequent doubling up of its outer layers along a zone of weakness. In the Appalachian Mountains, where the folds are prevailingly asymmetric, the evidence of this lateral compression is particularly apparent, for there the folds are not only overtipped in a constant direction, but they

are frequently torn asunder, and the overhanging limb of the fold is thrust over the recumbent limb.

In contrast to such mountains, where compressive stress in the earth's crust is the formative agency, we have mountain ranges of another type, which owe their existence to the rotation of long, narrow blocks of the earth's crust which have been dislocated one from another by parallel rupture planes. The elevated edge of such a tilted block becomes a mountain crest. On one side of the crest the slope is steep and corresponds to the rupture plane; on the other, the slope is gentle and corresponds to the general surface before it suffered disturbance. Mountains of this type are common in the region of the Great Basin, which lies between the Wasatch and the Sierra Nevada, and they are therefore referred to by geologists as of the Basin Range type. In the genesis of such ranges we detect no evidence of the operation of compressive stresses but rather the reverse, viz.: a distension of the crust. At least, the effects appeal to us as the result of unequal uplift, or of general uplift with unequal collapse accompanied by rotation.

Now, in none of the various types of mountains which we have thus passed briefly in review does the upbuilt or uplifted mass appear to us in the configuration due to the simple operation of the formative process. They have all been more or less affected by the operation of degradational forces. Under the general climatic conditions of our planet, whenever a mountain crest or peak has raised itself by any process whatsoever above the general level, it has thereby subjected itself to degradational attack, the intensity of which increases with the altitude and with the angular declivity of the elevated mass. Shasta is no longer a simple volcanic cone. Its slopes have been gullied and scored by the combined attack of the atmosphere, glaciers, and running water. The pile originally was not homogeneous. The combined forces of degradation have long since discovered

this, and the incisions made upon it have followed lines of least resistance. The degradation is therefore proceeding irregularly and is more or less expressive of the heterogeneity of the pile.

In the volcanic range of Salvador and Guatemala we find, to be sure, the cones of living volcanoes which are perfect in their formative profiles, but elsewhere in the same range, where the volcanoes are dormant or extinct, and accumulation has ceased to make good the ravages of the atmosphere, these profiles have been modified in a degree proportional to the time that has elapsed since the formative process stopped.

In the Andes many of the vast cones which constitute the culminating peaks have profiles referable solely to formative processes, because the volcanoes are either active or only recently extinct; but the mountain mass, upon which these individual and more or less isolated volcanoes are perched, has had its character, as an upheaved belt of the earth's crust, greatly modified by degradational processes. Deep cañons cut far into the heart of the mountain and the sculpture of the mass has become more prominent than the simple outlines of the original block. The same is true of the Caucasus. Elbruz, seated upon the broad upland of the range, has been in the formative stage until very recently and so retains the essential features due to the process of its upbuilding, while the Caucasus proper, pari passu with that upbuilding, has suffered severely from degradation. Great cañons trench the mountain ridge and subdivide what would otherwise have been a mass of simple outlines into an aggregate of plateaus, ridges, and peaks. The variety and complexity of forms due solely to sculpture bewilder the observer, absorb his attention, and usually preclude his recognition of the character of the primitive block from which such forms have been carved.