Darwin on Vegetable Mould.

In the year 1837, a short paper was read by me before the
Geological Society of London, {2} "On the Formation of Mould," in
which it was shown that small fragments of burnt marl, cinders,
&c., which had been thickly strewed over the surface of several
meadows, were found after a few years lying at the depth of some
inches beneath the turf, but still forming a layer. This apparent
sinking of superficial bodies is due... to the large
quantity of fine earth continually brought up to the surface by
worms in the form of castings. These castings are sooner or later
spread out and cover up any object left on the surface. I was thus
led to conclude that all the vegetable mould over the whole country has passed many times through, and will again pass many times through, the intestinal canals of worms. Hence the term "animal mould" would be in some respects more appropriate than that commonly used of "vegetable mould."

...nowhere in England have I seen the
ground so thickly covered with castings as on commons, at a height of several hundred feet above the sea. In woods again, if the loose leaves in autumn are removed, the whole surface will be found strewed with castings.
From: http://www.darwin-literature.com/The_Formation_Of_Vegetable_Mould/0.html