without fossils. Throughout the whole length of the chain, wherever
the junction of the Siwaliks with the pre-Tertiary rocks has been seen,
it is a great reversed fault. West of the Blas river a similar reversed
fault forms the boundary between the lower Tertiaries and the
pre-Tertiary rocks of the Himalaya, while between the Sutlej and
the Jumna rivers, where the lower Tertiaries help to form the lower
Himalaya, the fault lies between them and the Siwaliks. The hade
of the fault is constantly inwards, towards the centre of the chain,
and the older rocks which form the Himalaya proper, have been
pushed forward over the later beds of the sub-Himalaya. But the
fault is more than an ordinary reversed fault: it was, nearly everywhere,
the northern boundary of deposition of the Siwalik beds, and
only in a few instances do any of the Siwalik deposits extend even to
a short distance beyond it. The fault in fact was being formed
during the deposition of the Siwalik beds, and as the beds were laid
down, the Himalaya were pushed forward over them, the Siwaliks
themselves being folded and upturned during the process. Accordingly,
in some places the Siwaliks now form a continuous and conformable
series from base to summit, in other places the middle beds
are absent and the upper beds of the series rest upon the upturned and
denuded edges of the lower beds. The Siwaliks are fluviatile and
torrential deposits similar to those which are now being formed
at the foot of the mountains, in the Indo-Gangetic plain; and
their relations to the older rocks of the Himalaya proper were
very similar to those which now exist between the deposits of
the plain and the Siwaliks themselves. But the great fault just
described is not the only one of this character. There is a series of
such faults, approximately parallel to one another, and although
they have not been traced throughout the whole chain, yet wherever
they occur they seem to have formed the northern boundary of
deposition of the deposits immediately to the south of them. It
appears, therefore, that the Himalaya grew southwards in a series
of stages. A reversed fault was formed at the foot of the chain, and
upon this fault the mountains were pushed forward over the beds
deposited at their base, crumpling and folding them in the process,
and forming a sub-Himalayan ridge in front of the main chain.
After a time a new fault originated at the foot of the sub-Himalayan
zone thus raised, which now became part of the Himalaya themselves,
and a new sub-Himalayan chain was formed in front of the previous
one. The earthquakes of the present day show that the process is
still in operation, and in time the deposits of the present Indo-Gangetic
plain will be involved in the folds.
The regular form of the Himalaya, constituting an arc of a true circle, appears to indicate that the whole chain has been pushed forward as one mass upon a gigantic thrust-plane; but, if so, the dip of the plane must be low, for a line drawn along the southern foot of the Himalaya would coincide with the outcrop of a plane inclined to the surface at an angle of about 14°. The thrust-plane, then, does not coincide with any of the boundary faults already mentioned, which are usually inclined at angles of 50° or 60°. The latter are due to the fact that, although, perhaps, the whole mass above the thrust-plane may move, yet the pressure which pushes it forwards necessarily proceeds from behind. The back, accordingly, moves faster than the front, and the whole is packed together; as when an ice-floe drives against the shore, the ice breaks and the outer fragments ride over those within. The great thrust-plane which is thus imagined to exist at the base of the Himalaya, corresponds with the “major thrusts” of the N.W. Highlands of Scotland, and the reversed faults which appear at the surface with the “minor thrusts.” (P. La.)
Such is the general outline of Himalayan evolution as now understood, and the process of it has led to certain marked features of scenery and topography. Within the area of the trans-Indus mountains we have beds of hard limestone or sandstone alternating with soft shales, which leads to theTopographical results of evolution. scooping out by erosion of long narrow valleys where the shales occur, and the passage of the streams through deep rifts or gorges across the hard limestone anticlinals, which stand in irregular series of parallel ridges with the eroded valleys between. The great mass of the Himalaya exhibits the same structure, due to the same conditions acting for longer periods and on a much larger scale; but the structure is varied in the eastern portions of the mountains by the effect of different climatic conditions, and especially by the greater rainfall. Instead of wide, barren, wind-swept valleys, here are found fertile alluvial plains—such as Manipur—but for the most part the erosive action of the river has been able to keep pace with the rise of the river bed, and we have deep, steep-sided valleys arranged between the same parallel system of folds as we see on the western frontier, connected by short transverse gaps where the rivers cross the folds, frequently to resume a course parallel to that originally held. An instance of this occurs where the Indus suddenly breaks through the well-defined Ladakh range in the North-west Himalaya to resume its north-westerly course after passing from the northern to the southern side of the range. The reason assigned for these extraordinary diversions of the drainage right across the general strike of the ridges is that it is antecedent—i.e. that the lines of drainage were formed ere the folds or anticlinals were raised; and that the drainage has merely maintained the course originally held, by the power of erosion during the gradual process of upheaval.
In the outer valleys of the Himalaya the sides are generally steep, so steep as to be liable to landslip, whilst the streams are still cutting down the river beds and have not yet reached the stage of equilibrium. Here and there a valley has become filled with alluvial detritus owing to some local impediment in the drainage, and when this occurs there is usually to be found a fertile and productive field for agriculture. The straits of the Jhelum, below Baramulla, probably account for the lovely vale of Kashmir, which is in form (if not in principles of construction) a repetition on grand scale of the Maidan of the Afridi Tirah, where the drainage from the slopes of a great amphitheatre of hills is collected and then arrested by the gorge which marks the outlet to the Bara.
Other rivers besides the Indus and the Brahmaputra begin by
draining a considerable area north of the snowy range—the Sutlej,
the Kosi, the Gandak and the Subansiri, for example.
All these rivers break through the main snowy range ere
they twist their way through the southern hills to theGeneral Himalayan formation
is typical.
plains of India. Here the “antecedent” theory will not
suffice, for there is no sufficient catchment area north of
the snows to support it. Their formation is explained by a process
of “cutting back,” by which the heads of these streams are gradually
eating their way northwards owing to the greater rainfall on the
southern than on the northern slopes. The result of this process is
well exhibited in the relative steepness of slope on the Indian and
Tibetan sides of the passes to the Indus plateau. On the southern or
Indian side the routes to Tibet and Ladakh follow the levels of
Himalayan valleys with no remarkably steep gradients till they near
the approach to the water-divide. The slope then steepens with the
ascending curve to the summit of the pass, from which point it falls
with a comparatively gentle gradient to the general level of the
plateau. The Zoji La, the Kashmir water-divide between the
Jhelum and the Indus, is a prominent case in point, and all the passes
from the Kumaon and Garhwal hills into Tibet exhibit this formation
in a marked degree. Taking the average elevation of the central
axial line of snowy peaks as 19,000 ft., the average height of the
passes is not more than 10,000 owing to this process of cutting down
by erosion and gradual encroachment into the northern basin.
| S. | N. |
| |
| r = Recent; d = Upper siwalik conglomerate; c = Middle siwalik sandstones; b = Lower (Nahan) siwaliks; a = Nummulitic; n = Older rocks of Himalayas | |
| C S Middlemiss | |
| Section across the sub-Himalayan zone. | |
Meteorology.—Independently of the enormous variety of topographical conformation contained in the Himalayan system, the vast altitude of the mountains alone is sufficient to cause modifications of climate in ascending over their slopes such as are not surpassed by those observed in moving from the equator to the poles. One half of the total mass of the atmosphere and three-fourths of the water suspended in it in the form of vapour lie below the average altitude of the Himalaya; and of the residue, one-half of the air and virtually almost all the vapour come within the influence of the highest peaks. The regular variations in pressure of the air indicated by the barometer and the annual and diurnal oscillations are as well marked in the Himalaya as elsewhere, but the amount of vapour held in suspension diminishes so rapidly with the altitude that not more than one-sixth (sometimes only one-tenth) of that observed at the foot of the mountains is found at the greatest heights. This is dependent on the temperature of the air which rapidly decreases with altitude. On the mountains every altitude has its corresponding temperature, an elevation of 1000 ft. producing a fall of 312°, or about 1° to each 300 ft. The mean winter temperature at 7000 ft. (which is about the average height of Himalayan “hill stations”) is 44° F. and the summer mean about 65° F. At 9000 ft. the mean temperature of the coldest month is 32° F. At 12,000 ft. the thermometer never falls below freezing-point from the end of May to the middle of October, and at 15,000 ft. it is seldom above that point even in the height of summer. It should be noted that the thermometrical conditions of Tibet vary considerably from those of the Himalaya. At 12,000 ft. in Tibet the mean of the hottest month is about 60° F. and of the coldest about 10° F. whilst, at 15,000 ft. the frost is only permanent
