Page:Popular Science Monthly Volume 19.djvu/440

426 ocean-shore, so as to reveal its existence. The surface covering of the ice was the surface of the country, and, over many miles north from its actual termination, it supported a varied and even rank vegetation." Professor Winchell observes that the facts reported by Mr. Dall throw light on the manner of formation and deposit of the till, and on the origin of kames. The kames are gravel-ridges lying in till-covered countries, occupying the lower situations and generally bordered on either side by a parallel strip of swamp or low land. Now, if we suppose that the till before its deposit lay on the surface of the ice, it is plain that the surface drainage, gathering into streams, would produce deep channels in the ice-sheet, in the bottom of which would be gathered such stones and gravels as the stream could not carry away, and these would gradually sink deeper into the ice, perhaps to the rocky floor itself. When the ice had entirely disappeared, the bed of coarser matters thus formed "would lie undisturbed in its beautiful stratification, where the river produced it"; while on either side would be first the swampy or low land produced by the wash of the stream, and outside of this the unmodified till.

Undergrowth and Forest-Trees.—M. Gourmand has recently described some observations which he has made on the influence of thickets upon the decomposition of vegetable matter and the growth of large trees. A thicket may be formed in the course of eight or ten years after the undergrowth has started; as it rises in height we can at last distinguish between the atmosphere beneath it and the superior atmosphere to which the tops of the larger trees are exposed. Seventeen years of watching and periodical measuring of the growth of the trees of a tract bearing a deciduous undergrowth and a larger coniferous growth have shown that the rate of growth of the larger trees diminishes as the undergrowth becomes more dense; the only exceptions arc in glades where the undergrowth sends up vertical limbs instead of spreading out sidewise. The rate of growth thus appears to be modified according as the light is or is not able to penetrate the depth of the wood, and, as carbonic acid is in a corresponding degree more or less rapidly formed from the decomposition of the substances composing the humus. M. Gourmand concludes from these observations that light, when it reaches the ground after passing through foliage, stimulates the production of carbonic acid in decompositions that engender humus in proportion as that gas is decomposed by the green parts; that the growth of the larger trees is retarded, although their green parts stand out in full air and light, where the lower thicket cuts the light off too much from the soil and diminishes its reflex action on their tops; that this effect is governed largely by the arrangement of the limbs of the undergrowth, as it is less marked in glades, where they take a vertical direction; and that the humus under too dense an undergrowth loses a part of its efficiency, and presents an analogy with barn-yard manure, which will remain inert for several years if it is buried too deeply.

The Weather, and Summer Diarrhœas.—Mr. G. B. Longstaff has recently pointed out, in an address before the Society of Medical Officers of Health, some facts concerning the prevalence of summer diarrhœa in England, which are not fully accounted for by the prevailing theories of its origin. He distinguishes between two kinds of diarrhœa: one general, prevailing throughout the year, affecting persons at all ages, and nearly evenly distributed in town and country; and a specific form, which prevails in the summer months and affects most persons at the extremes of life, particularly infants of less than two years old, and which is not definitely modified by changes of season. The second form is, as a rule, a disease of towns, but different towns are differently affected by it. The summer of 1880 was a warm one in England, with the mean temperature in August and September above the average, and a high rainfall in July and September, while little rain fell in August; the death-rate from diarrhœa in England and Wales exceeded the average of the previous ten years by nearly fifty per cent. The comparison of the mortality from this cause in different towns, as between the towns and with the general mortality of the kingdom, failed to establish