HIMALAYA, that portion of the mountain region between India and Tibet enclosed within the arms of the Indus and the Brahmaputra, having, therefore, a length of 1,50o m., and a width from Ioo to 15o miles. North-west of the Indus the region of mountain ranges which stretches to a junction with the Hindu Kush, south of the Pamirs, is usually known as Trans-Himalaya. Thus the Himalaya represents the southern face of the great central elevated region—the plateau of Tibet—the northern face of which is buttressed by the Kuen Lun.
(I) The Great Himalayas.—The main ranges, which lie in the north, rise above the snow line and have an average elevation of 20,000 ft. above the sea. Here occur the highest peaks, e.g., Everest, K2 (Godwin Austen), Kanchinjanga, etc.
(2) The Lesser Himalayas.—The middle ranges, which are closely related to and lie south of the Great Himalayas. They form an intricate mountain system, with an average height of 12,000 to 15,000 ft. above the sea.
(3) The Outer Himalayas.—These comprise the Siwalik ranges, lie between the Lesser Himalayas and the plains, and have an average height of 3,00o to 4,00o ft. above the sea.
Although the northern limits of the Tsanpo basin are not sufficiently well known to locate the Indo-Tibetan watershed even approximately, there exists some evidence of the nature of that strip of Northern Himalaya on the Tibeto-Nepalese border which lies between the line of greatest elevation and the trough of the Tsanpo. Recent investigations show that all the chief rivers of Nepal flowing southwards to the Tarai take their rise north of the line of highest crests, the "Great Himalayas," and that some of them drain long lateral high-level valleys enclosed between minor ridges whose trend is parallel to that of the Himalayas and, oc casionally, almost at right angles to the course of the main drain age channels breaking down to the plains. This formation brings the southern edge of the Tsanpo basin to the immediate neigh bourhood of the banks of that river, which runs at its foot like a drain flanking a wall. North of Bhutan, between the Himalayan crest and Lhasa, this formation is approximately maintained; farther east, although the same natural forces first resulted in the same effect of extensive curves of ridge and furrow, the abundant rainfall and the totally distinct climatic conditions which govern the processes of denudation subsequently led to the erosion of deeper valleys enclosed between forest-covered ranges which rise steeply from the river banks.
Geologically, the Himalaya may be divided into three zones. The northern (Tibetan zone), in which fossiliferous beds of the Palaeozoic and Mesozoic age are largely developed—excepting in the north-west, no such rocks are known on the southern flanks. The second (Himalayan zone) comprises most of the Lesser and Great Himalayas, and is composed chiefly of crystalline and metamorphic rocks, together with unfossiliferous sedimentary beds supposed to be of Palaeozoic age. The southern (sub Himalayan zone) consists entirely of Tertiary beds, and especially of the Upper Tertiaries. The oldest beds which have yielded fossils occur in the Spiti valley and belong to the Cambrian system (the "Haimanta" system). These are underlain by Archaean gneisses. There appears to be no break in the Lower Palaeozoic beds in the Spiti region but in other parts of the Central Hima layas the conglomerate at the base of the Permian rests uncon formably upon older formations. This conglomerate forms an im portant datum line. From the Permian to the Lias the sequence in the central Himalaya shows no sign of a break. The Spiti shales follow, and although they contain Middle and Upper Jurassic fossils no break has yet been proved at their base. The Spiti shales are succeeded conformably by Cretaceous beds (Giumal sandstone below and Chikkim limestone above), and these are followed without a break by Nummulitic beds of Eocene age. The beginning of the Tertiary period was marked by violent igneous activity, in which intrusion and extrusion occurred. The next succeeding deposit is a sandstone, often highly inclined, which rests unconformably upon the Nummulitic beds and re sembles the Lower Siwaliks of the sub-Himalaya (Pliocene) but which as yet has yielded no fossils of any kind. The whole is overlaid unconformably by the younger Tertiaries of Hundes, which are perfectly horizontal and have been folded.
It is evident that in the northern part of the Himalayan belt, at least in the Spiti area, there can have been no post-Archaean folding of any magnitude until after the deposition of the Nummu litic beds, and that the folding was completed before the later Tertiaries of Hundes were laid down. It was, therefore, during the Miocene period that the elevation of this part of the chain began, while the disturbance of the Siwalik-like sandstone indi cates that the folding continued into the Pliocene period. Along the southern flanks of the Himalaya the history of the chain is still more clearly shown. The sub-Himalaya are formed of Ter tiary beds, chiefly Siwalik or Upper Tertiary, while the Lower Himalaya proper consist mainly of pre-Tertiary rocks 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 (the "Main Boundary Fault"). The bade 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 younger beds of the sub-Himalaya. Moreover, nearly everywhere the fault formed the northern boundary of deposition of the Siwalik beds, and only in a few instances do any of these deposits extend even to a short distance beyond it. The fault, in fact, was being formed by the deposition of the Siwalik beds, and as they were laid down, the Himalaya were pushed forward over them, so that they were folded and upturned during the process. 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. The "Main Boundary Fault" is really one of a series of approxi mately parallel faults, all of which formed the northern boundary of deposition of the deposits immediately south of them. 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. This process was repeated several times and the earthquakes of to-day in this region can be traced to the fault lines and show that crustal equilibrium has not yet been reached.
Topographical Results of Evolution.—The uplift of the Himalayas was a gradual process protracted over a very long period and the process had a very marked effect upon the scenery, the topography and the river system. The latter is not consequent upon the structure, but the principal rivers were of an age anterior to the Tertiary earth-movements and the drainage is spoken of as "antecedent." During the slow process of uplift, folding and faulting the rivers were able to keep, for the most part, to their original courses, although their erosive power was increased owing to increased gradients. Thus we find the rivers cutting through the main chains in deep transverse gorges after flowing for long distances parallel to the trend of the chain. Examples of this are the Indus and the Brahmaputra.
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 the 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 steep ness 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 re markably 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. 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 io,000 owing to this process of cutting down by erosion and gradual en croachment into the northern basin. (See also TIBET; LEH.) Climate.—Independently of the enormous variety of topog raphy the vast altitude of the mountains alone is sufficient to cause very great modifications of climate. 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 mean winter temperature at 7,00o ft. (which is about the average height of Himalayan "hill stations") is 44° F and the summer mean about 65 ° F, but in the valleys a temperature of 90 to ioo° F may be felt during the day in May and June. At 9,00o ft. the mean temperature of the coldest month is 3 2 ° F. At 12,000 ft. the thermometer remains above freezing-point from the end of May to the middle of October, but at 15,000 ft. is seldom above that point even in the height of summer. It should be noted that the thermometrical conditions of Tibet vary con siderably from those of the Himalaya. At 12,000 ft. in Tibet the mean of the hottest month is about 6o° F and of the coldest about io° F, while at 15,000 ft. the frost is only permanent from Nov. I to May 1. In Tibet the daily range may be 6o° and often reaches 50°. The temperature of the sandy surface varies even more and records of 2° and 96° have been taken within 24 hours by the Everest expedition. The soil beneath the surface is far more constant in temperature. The difference of temperature between forest-clad ranges and the Indian plains is twice as much in April and May as in December or January; and the difference between the temperature of a well-wooded hill top and the open valley below may vary from 9° to 24° within twenty-four hours. The general relations of temperature to altitude as determined by Himalayan observations are as follows: (I) The decrease of temperature with altitude is most rapid in summer. (2) The an nual range diminishes with the elevation. (3)The diurnal range diminishes with the elevation.
Rainfall is chiefly related to the monsoonal rainfall of India of which the Himalayas catch a large portion. There is more rain, and rain for a longer season, in the east than in the west. The east may have so to 8o in. of rain against southward hill slopes and in certain spots even higher totals are reached, for example Darjeeling gets i 20 inches. The rainfall diminishes west ward to 4o inches near the place where the Ganges leaves the mountains and falls to 3o towards the emergence of the Indus from the mountains, while in places in a rain-shadow the fall may be only 15 in. though they are in a zone that on the whole averages 30 or more. In the arid zone protected from the rainy winds the rainfall goes below 15 in. and in Tibet for the most part well below 10 inches.
In Tibet the rapid heating of the plains by the morning sun brings a fierce wind down daily from the eternal snows and some air may thus drift through such partial gaps as there are in the great range from India to the Tibetan table-land.
In the eastern Himalaya the ordinary winter limit of snow is 6,000 ft. and it never lies for many days even at 7,000 ft. In Kumaon, on the west, it usually reaches down to the 5,00o ft. level and occasionally to 2,500 feet. Snow has been known to fall at Peshawar. At Leh, in western Tibet, hardly 2 ft. of snow are usually registered and the fall on the passes between 17,000 and 19,000 ft. is not generally more than 3 f t., but on the Himalayan passes farther east the falls are much heavier. Even in September these passes may be quite blocked and they are not usually open till the middle of June. The snow-line, or the level to which snow recedes in the course of the year, ranges from 15,000 to 16,000 ft. on the southern exposures of the Himalaya that carry per petual snow, along all that part of the system that lies between Sikkim and the Indus. It is not till December that the snow begins to descend for the winter, although after September light falls occur which cover the mountain sides down to 12,000 ft., but these soon disappear. On the snowy range the snow-line is not lower than 18,50o ft. and on the summit of the table-land it reaches to 20,000 ft. On all the passes into Tibet vegetation reaches to about 17,500 ft., and in August they may be crossed in ordinary years up to 18,400 ft. without finding any snow upon them ; and it is as impossible to find snow in the summer in Tibet at 15,5oo ft. above the sea as on the plains of India.
The glaciers of the Himalayas seem to be in retreat. On the north side of Everest the Rongbuk glacier ends at about 16,500 ft. and the Kyetrak glacier at 15,400 feet. In the Kinchinjunga group the glaciers may come down to 13,000 ft. while in Kumaon they reach 12,000 ft. and in Kashmir, in special situations, they may come as low as 8,000 feet. The level reached is, as in Europe, 3,000-5,000 ft. below the limit of perpetual snow. The air tem peratures at the ends of the glaciers are about as in Europe, namely, nearly 6o° in July and diminishing slowly until the end of September. Several glaciers are 11-16 m. long, the higher figure being attained in the Kinchinjunga series. Italian geologists have investigated the moraines of the Karakoram Mts. and have given evidence of the great ice sheets of the Pleistocene glacia tion, ice sheets which may have been one continuous mass over the whole of central Asia. They have further shown that there is evidence of phases of variation during the Pleistocene Ice Age which seem to correspond with those observed in Europe.
The European flora of the Mediterranean reaches the Hima laya, but very few of its members attain the eastern end of the chain. Some Japano-Chinese forms such as Aucuba, Helwingia, Skimmia, Adamia, Goughia reach the east of the chain, but do not spread far to the west. It is naturally at fairly high levels that the European forms are found, but Tilia, Fagus, Arbutus, Erica, Azalea and the Cistaceae are absent. An African element is claimed to reach the western Himalayas.
The eastern section is very warm and damp and intertropical plants live even at 7,000 ft. above sea, while the forest reaches up to nearly 13,000 ft. in places. In the west the corresponding levels are 5,000 and under 12,000 ft., the dry season being more marked and the variation of temperature greater.
Using Osmaston's general classification of the forests we have: (I) The Shorea (-Sal) Anogeissus-Pinus formation in the warm moist valleys, and up to about 6,50o f t. in places. The warm temperature implies heavy evaporation and rapid decomposition of humus. This forest may be called warm subtropical, the trees form an open canopy and belong to species that either shed their leaves in the warm weather before the rains, or are needle-leaved conifers. Pinus longi f olia is specially able to survive fires and Anogeissus has special powers of coppicing so that it survives graz ing and lopping. Anogeissus lives up to a 4,000-ft. level in large groups and reaches west as far as the neighbourhood of Srinagar (Kashmir). (2) The Quercus-Abies formation on the moist hill sides from 5,00o ft. to 11,500 f t., where the rainfall is typically 50-8o in. per annum, and some of it is snow. The parts which face south are apt to get dry in November and December and from April to June and in the latter season there are often forest fires. The heat and rainfall prevent accumulation of much humus on the south side except under special circumstances, but the north side has humus, and there forest fires are rare. The general cover ing is dense on the north, but less so on the south; evergreen oaks are the commonest trees, epiphytic ferns abound and on higher levels mosses and lichens encrust the tree-trunks. (3) The Pinus-Cedrus formation on the drier mountain flanks from 9,000 ft. to 12,00o ft. with a rainfall of i o to 4o in., but in winter the precipitation is mainly snowfall and the temperature is low. Slopes facing the sun are apt to be very dry and suffer from fires. Trees and grass often occur together, both giving place to shrubs in some areas with long dry periods. Cedrus itself reaches the 8,000 ft. level in the moist zone, but the i o,000-f t. level in the dry one, it grades into cypress below and blue-pine above. The cypress suffers specially from fires and perhaps for this reason is rarely found near grassland in the Himalayas ; it favours rather the steep rocky hill sides. (4) The Betula-Rhododendron formation has a general distribution between the 9,000- and the 13,500-ft. contours with vegetative activity chiefly during the monsoon rains. The cold soil makes absorption by the roots slow even if there be abundant moisture, as there is in some parts. The rainfall varies very much and a large proportion is really snowfall, and the formation generally occurs where insolation is low because of a north aspect, or cloudiness. The formation makes a dense cover ing, but there are no large trees and the branches often bend down and are flexible. It is sometimes quite absent in the moist zone in which oak forest may be succeeded above by grass; much has to be allowed in this matter for the depredations of grazing animals. Sometimes one finds a Rhododendron-Lonicera shrubby group, sometimes a Betula-Abies tree-group, but it is difficult to distinguish different conditions for them. Lonicera obovata is restricted to the higher part of the zone. (5) The Caragana Lonicera-Artemisia formation in the arid Tibetan zone 9,00o ft. to 15,000 f t. above sea with extremely dry conditions during the short period in which vegetative activity is possible. The sun is strong and the dry atmosphere gives it unusual power. The strong daily wind down from the peaks transports sand and dust. Trees and shallow rooted grasses are almost absent, shrubs have branches bent down and very flexible and the leaves are deciduous. Salix and Myricaria form associations related to No. 5, but choos ing moister spots near streams. Only a few plants, save willows and junipers, grow taller than 2 ft. or so. The small amount of grass, and shoots of shrubs are a precious resource to yaks and other animals of the heights and water weeds of the cold lakes are also useful to them. The mouse-hares lay up stores of seeds in their burrows for the winter and perhaps on the whole help in this way to maintain some species of plants.
The bird life of the Himalayas is wonderfully rich. Eagles, vul tures and many birds of prey soar to great heights. Gorgeous colouring occurs among sun-birds, trogons, kingfishers, etc., but of the parrot family only parrakeets and a small lory are known. The peacock occurs in the forests bordering the plains on the Indian side, and pheasants and partridges include varieties adapted to life on the snow at great altitudes. Waders and waterfowl are not very important features.
Among the snakes the python is found on the Indian side and the cobra penetrates as high as some 8,000 ft. above sea. Lizards and frogs abound and reach remarkable elevations, the lizard, Phrynocephalus, and a frog being found even in Tibet. Of the fishes only two Acanthopterygii enter the mountains, most of the fish of the region belonging either to the catfish family (Siluridae) or the carp family (Cyprinidae), a few of the former and a number of the latter live in Tibetan waters. Most of the fishes of high mountain-torrents have suckers with which to hold on to rocks. The salmon family is absent from the region.
The butterflies are renowned for their magnificence and include among others Papilionidae, Nymphalidae, Morphidae, and Da naidae. Insect life of all kinds is very rich and locust-swarms are carried up by the winds even to the perpetual snow.
Major Hingston of the Indian Medical Service has given valu able accounts of the natural history of animals at great altitudes in the Himalayas. Protective colouring, usually in browns and greys akin to those of the desert-like surface of Tibet, is almost universal. The mouse-hare (Ocliotona) is common and even the sheep and gazelles are coloured like the soil. Most of the birds are protectively coloured, though some show bright colours when on the wing. Unlike the large mammals, the large birds are not protected in colour, and the eagles, kites, ravens and choughs abound. It is said that the birds living among the villages are not protectively coloured. That type of coloration is common among insects, spiders, etc.
The intense cold of the great heights is made still more trying by the fierce winds that rush down from the snows to the plateau as the latter warms up in the morning sun. As a protection most mammals have thick hair and the dogs have wool as well ; even the pig is thickly covered with hair. The yak has a coat of wool over which is the long hair that forms a protective skirt under the belly, while the tail may be described as a unique windscreen. The hair under the belly is thought by Major Hingston to keep the animal's legs warm when it is resting. The yak stands with its hindquarters to the wind. The smaller animals and the birds mostly shelter among the rocks and stones when the wind be comes strong, but many of them illustrate its power of damaging fur and feathers. The birds generally face the wind and the great vultures are able to some extent to make use of it. The butterflies may protect themselves by pressing their wings against a stone in as sheltered a nook as can be found. Major Hingston found Parnassius sp. as high as 17,000 ft. above sea-level.
The sharp changes of temperature within a few hours, known in all mountain regions, are accentuated here and may be very trying in winter. Many animals of the high mountains are pro tected from this by the habit of hibernation in burrows where, a little beneath the snow and the surface soil, the temperature is much more uniform. The mouse-hare was seen by Major Hingston as high as 17,000 ft. above sea and he thinks it may reach 20,000 ft. as a stray. Several birds go up to 20,000 ft. and more on the mountain side, and choughs followed the Everest climbers as high as 27,000 ft.; they seem least incommoded by the great heights. The habits of mammals and birds have been noticed to change on the great heights; thus birds which elsewhere are of the wild, live there on and around the villagers' houses, while communities (even some of birds and mammals) are a characteristic develop ment with mutual aid in view.
See D. N. Wadia, Geology of India (i919) ; G. Dainelli, Relazione Scientifiche della Spedizione Italiana De Filippi nell' Himalaia, Cam corum e Turchestan Cinese (1913—i4) Serie II. Resultati Geologici e Geografici, 10 vols.; E. Argand, La Tectonique de l'Asie (Liege, 1924) ; A. E. Osmaston, "Forest Communities of the Garhwal Himalayas," Ecology (Cambridge, 1922) ; E. Trinkler, "Tibet" Mitt. Geogr. Ges. Munchen Bd. 15, 1922 and the literature of the Everest Expedi tions.