第55章

Dr.King informs me that the majority of the before described gigantic castings, which he found on a fully exposed, bare, gravelly knoll on the Nilgiri Mountains in India, had been more or less weathered by the previous north-east monsoon; and most of them presented a subsided appearance.The worms here eject their castings only during the rainy season; and at the time of Dr.King's visit no rain had fallen for 110 days.He carefully examined the ground between the place where these huge castings lay, and a little watercourse at the base of the knoll, and nowhere was there any accumulation of fine earth, such as would necessarily have been left by the disintegration of the castings if they had not been wholly removed.He therefore has no hesitation in asserting that the whole of these huge castings are annually washed during the two monsoons (when about 100 inches of rain fall) into the little water-course, and thence into the plains lying below at a depth of 3000 or 4000 feet.

Castings ejected before or during dry weather become hard, sometimes surprisingly hard, from the particles of earth having been cemented together by the intestinal secretions.Frost seems to be less effective in their disintegration than might have been expected.Nevertheless they readily disintegrate into small pellets, after being alternately moistened with rain and again dried.Those which have flowed during rain down a slope, disintegrate in the same manner.Such pellets often roll a little down any sloping surface; their descent being sometimes much aided by the wind.The whole bottom of a broad dry ditch in my grounds, where there were very few fresh castings, was completely covered with these pellets or disintegrated castings, which had rolled down the steep sides, inclined atan angle of 27 degrees.

Near Nice, in places where the great cylindrical castings, previously described, abound, the soil consists of very fine arenaceo-calcareous loam; and Dr.King informs me that these castings are extremely liable to crumble during dry weather into small fragments, which are soon acted on by rain, and then sink down so as to be no longer distinguishable from the surrounding soil.He sent me a mass of such disintegrated castings, collected on the top of a bank, where none could have rolled down from above.They must have been ejected within the previous five or six months, but they now consisted of more or less rounded fragments of all sizes, from 0.75 of an inch in diameter to minute grains and mere dust.Dr.King witnessed the crumbling process whilst drying some perfect castings, which he afterwards sent me.Mr.Scott also remarks on the crumbling of the castings near Calcutta and on the mountains of Sikkim during the hot and dry season.

When the castings near Nice had been ejected on an inclined surface, the disintegrated fragments rolled downwards, without losing their distinctive shape; and in some places could "be collected in basketfuls." Dr.King observed a striking instance of this fact on the Corniche road, where a drain, about 2.5 feet wide and 9 inches deep, had been made to catch the surface drainage from the adjoining hill-side.The bottom of this drain was covered for a distance of several hundred yards, to a depth of from 1.5 to 3 inches, by a layer of broken castings, still retaining their characteristic shape.Nearly all these innumerable fragments had rolled down from above, for extremely few castings had been ejected in the drain itself.The hill-side was steep, but varied much in inclination, which Dr.King estimated at from 30 degrees to 60 degrees with the horizon.He climbed up the slope, and "found every here and there little embankments, formed by fragments of the castings that had been arrested in their downward progress by irregularities of the surface, by stones, twigs, &c.One little group of plants of Anemone hortensis had acted in this manner, and quite a small bank of soil had collected round it.Much of this soil had crumbled down, but a great deal of it still retained the form of castings." Dr.King dug up this plant, and was struck with the thickness of the soilwhich must have recently accumulated over the crown of the rhizoma, as shown by the length of the bleached petioles, in comparison with those of other plants of the same kind, where there had been no such accumulation.The earth thus accumulated had no doubt been secured (as I have everywhere seen) by the smaller roots of the plants.After describing this and other analogous cases, Dr.King concludes: "I can have no doubt that worms help greatly in the process of denudation."Ledges of earth on steep hill-sides.--Little horizontal ledges, one above another, have been observed on steep grassy slopes in many parts of the world.The formation has been attributed to animals travelling repeatedly along the slope in the same horizontal lines while grazing, and that they do thus move and use the ledges is certain; but Professor Henslow (a most careful observer) told Sir J.Hooker that he was convinced that this was not the sole cause of their formation.Sir J.Hooker saw such ledges on the Himalayan and Atlas ranges, where there were no domesticated animals and not many wild ones; but these latter would, it is probable, use the ledges at night while grazing like our domesticated animals.A friend observed for me the ledges on the Alps of Switzerland, and states that they ran at 3 or 4 ft.one above the other, and were about a foot in breadth.They had been deeply pitted by the feet of grazing cows.Similar ledges were observed by the same friend on our Chalk downs, and on an old talus of chalk-fragments (thrown out of a quarry) which had become clothed with turf.