These graves are, some of them, plainly quite new.

Some, again, are very old; for trees of all sizes are growing in them and over them.
What makes them?
A question not easily answered.

But the shrewdest foresters say that they have held the roots of trees now dead.

Either the tree has fallen and torn its roots out of the ground, or the roots and stumps have rotted in their place, and the soil above them has fallen in.
But they must decay very quickly, these roots, to leave their quite fresh graves thus empty: and--now one thinks of it--how few fallen trees, or even dead sticks, there are about.

An English wood, if left to itself, would be cumbered with fallen timber; and one has heard of forests in North America, through which it is all but impossible to make way, so high are piled up, among the still- growing trees, dead logs in every stage of decay.