OCR Text |
Show 158 GHEAT STONES CHAP. III. step of the process could be followed, fr01n tho accidental deposition of a single casting on a small obj ct lying loose on the surface, to its Leiner ontancrled amidst the matted roots of b 0 the turf, and lastly to its being embedded in the mould at various depths beneath tho surface. When the smne field was re-examined after the interval of a few years, suc;h objects were found at a greater depth tl1an before. The straightness and regularity of the lines formed by the embedd d objects, and their parallelism with the surface of the land, are the most striking features of the case; for this parallelisn1 shows how equably the worn1s must have worked; the result being, however, partly the effect of the washing down of the fresh castings by rain. Tho specific gravity of the objects does not affect their rate of sinking, as could be seen by porous cinders, burnt marl, chalk and quartz pebbles, having all sunk to the same depth within the same time. Considering the nature of the substratum, which at Leith Ilill Place was sandy soil including many bits of rock, and at Stonehenge, chalk-rubble with broken fEnts; considering, also, the presence of the turf-covered sloping border of moulcl CIIAP. III. UNDERMINED BY WOHMS. 159 round the great fragments of stone at both these places, their sinkiug does not appear to have been sensibly aided by their weight, though this was considerable.* On the number of worms which live within. a given space.-We will now show, firstly, what a vast number of worms live unseen by us beneath our feet, and, secondly, the actual weight of the earth which they bring up to the surface within a given space and within a given time. Hensen, who has published so full and interesting an account of the habits of worn1s,t calculates, from the number which he found in a measured space, that thoro mu t exist 133,000 living worms in a hectare of land, or 53,7 67 in an acre. This latter nuinber of worms would weigh 356 pounds, taking Hensen's standard of the weight of a single worm, namely, one gram. It s11ould, however, be noted that this calculation is * Mr. R. Mallet remarks(' Quarterly Journal of Gcolog. 'oc.' vol. xxxiii., 1877, p. 745) that" tho extent to which the ground beneath the foundations of po1111erous architectural structures, such as cathedral towers, bas been known to become compressed, is as remarkable as it i~ instructive and curious. 'l'he amount of deprcs~:;ion in some cases may be measured by fee t." lie instances Lhe rl'owcr of Pisa, but auus that it was founded on "dense clay." t 'Zeit~:;chrift fUr wissensch. Zoolog.' Bu. xxviii., 1877, p. 354. |