Page:Popular Science Monthly Volume 20.djvu/308

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full of fragments of burned marl, conspicuous from their red color, fragments of coal-cinders, and a few white-quartz pebbles. Beneath this layer, and at a depth of four and a half inches from the surface, the original black, peaty, sandy soil with a few quartz pebbles was

PSM V20 D308 Vegetable mold used in fertilization.jpg
Fig. 3.—Section, reduced to Half the Natural Scale, of the Vegetable Mold in a Field, drained and reclaimed Fifteen Years previously. A. turf; B, vegetable mold without any stones; C, mold with fragments of burned marl, coal-cinders, and quartz-pebbles; D, sub-soil of black, peaty sand, with quartz-pebbles.

encountered. "Here, therefore," says Mr. Darwin, "the fragments of burned marl and cinders had been covered in the course of fifteen years by a layer of vegetable mold, only two and a half inches in thickness, excluding the turf." Six and a half years afterward this field was re-examined, and the fragments were found at from four to five inches beneath the surface, having been covered in that time with an inch and a half more of mold. The average annual increase of thickness for the whole period was ·19 of an inch. This was less than the average increase of thickness in some other fields similarly observed, in which the accumulation amounted to ·21 and ·22 of an inch annually.

Another field, lying upon the chalk, and sloping rather steeply in one part, which was turned into pasture-land in 1841, was for several years so thickly covered with small and large flints that Mr. Darwin's sons always called it "the stony field." When they ran down the slope the stones clattered together; and Mr. Darwin remembers doubting whether he should live to see the larger flints covered with vegetable