Proceeding to the principal scene of Dr. Hooker's own researches, we find that there are two secondary considerations which materially affect the melting of snow, and therefore exert a material influeuce on the elevation of the snow-line, but which have not been sufficiently dwelt upon, though they bear directly upon the great altitude of that line in the most elevated regions. From the imperfect transmission of the heating rays of the sun through films of water, it follows that tho direct effects of the rays, in clear sunshine, are very different at equal elevations of the moist outer and dry inner ranges of the Himalaya. Secondly, naked rock and soil absorb much more heat than surfaces covered with vegetation, and this heat, radiated from them again, is much more rapidly absorbed by the white snow than the direct heat of the sun's rays is. Hence, at equal elevations, the ground heats sooner, and the snow is more exposed to the heat thus radiated in arid Tibet than in the wooded and grassed mountains of Sikkim. In the latter region, "the position and elevation of the perpetual snow vary with those of the individual ranges, and their exposure to the south wind. The expression that the perpetual snow lies lower and deeper on the southern slopes of the Himalayan mountains than on the northern conveys a false impression. It is better to say that the snow lies deeper on the southern faces of the individual mountains and spurs that form the snowy Himalaya. The axis itself of the chain is generally far north of the position of the spurs that catch all the snow, and has comparatively little snowon it, most of what there is lying upon north exposures. ' Thus appears to be at last explained the apparent anomaly that the snow-line ascends in advancing north to the coldest Himalayan region; the position of the greatest peaks and of the greatest mass of perpetual snow being generally assumed, though erroneously, as indicating a ridge and watershed. "Travellers arguing from single mountains alone, on the meridional ridges, have at ono time supported and at another denied the assertion, that the snow lies longer and deeper on the north than on the south slope of the Himalayan ridge." The enormous accumulation of snow in Sikkim at 15,000 feet exercises a decided influence on the vegetation, preventing its exten sion upwards, which, in other situations, takes place to 16,000 and 17,000 feet. Glaciers descend to 15,000 feet in the tortuous gorges which immediately debouch from the snows of Kinchinjunga, but no plants grow on the debris they carry down, nor is there any sward of am or herbage at their base, the atmosphere Immediately around king chilled by the snows, and the summer sun rarely warming the soil. In the vicinity of Kinchinjhow, 21,000 feet high, and where the mean level of perpetual snow is 19,000 feet, coinciding, probably, with a mean temperature of 20°, the glaciers do not descend below 16,000 feet, but a green award of vegetation creeps up to their bases, and herbs grow on the patches of earth they carry down, while dwarf rhododendrons cover the moraines. Dr. Hooker concludes on this
subject with the following general statement :--" Looking eastward or westward on the map of India, we perceive that the phenomenon of snow is regulated by the same laws. From the longitude of Upper Assam in 95' F.. to that of Kashmir in 75' E., the lowest limit of perpetual snow is 15,500 to 16,000 feet, and a shrubby vegetation affects the most humid localities near It, at 12,000 to 14,000 feet. Receding from the plains of India and penetrating the mountains, the climate becomes drier, the snow-line rises, and vegetation diminishes, whether the elevation of the land increases or decreases; plants reaching 17,000 and 13,000 feet, and the snow-line 20,000 feet.. To mention extreme cases : the suow-level of Sikkim in latitude 27' 30' is at 16,000 feet, whereas in latitude 85° 30' Dr. Thomson found the snow-line 20,000 feet on the mountains near the Karakoram pass, and vegetation up to 18,500 feet—features I found to be common also to Sikkim in latitude 28"." The progress of physical geology brings before us, from time to time, the conditions of the earth's surface at former periods. An example of this, probably involving the consideration of the great extension of snow and ice in the northern hemisphere, southward from the arctic regions, at what has been termed the glacial epoch, as alluded to in several former articles, is stated by Dr. Hooker nearly in the following terms :—Were the snow-level in the Dingcham province of Tibet as low as it is in Sikkim, or 15,000 feet, the whole of Tibet, from the Donkia mountain northwards to the Yarn-Tsampu river, the average direction of which is west and east nearly in the latitude of 29' 10', "would be everywhere intersected by glaciers and other impassable barriers of snow and ice, for a breadth of fifty miles, and the country would have no parallel for amount of snow beyond the Polar circles. It is impossible to conjecture what would have been the effects on the climate of northern India and central Asia under these conditions. When, however, we reflect upon the evidences of glacial phenomena that abound in all the Himalayan valleys at and above 9000 feet elevation, it is difficult to avoid the conclusion that such a state of things once existed, and that at a comparatively very recent period." Valuable information on this subject, as well as on the philosophy of the snow-line as at present existing, will be found in Mr. W. If opkina's paper, "On the causes of changes of climate," ' Quart. Journ. of Geo. Soc.,' voL viii., p. 76-87.