In England ninety-four per cent of these stones found in this
bowlder-clay are "stranger" stones; that is to say, they do not
belong to the drainage area in which they are found, but must have
been carried there from great distances.

But how about the markings, the _striæ_, on the face of the
surface-rocks below the Drift? The answer is plain. _Débris_, moving
at the rate of a million miles an hour, would produce just such
markings.

Dana says:

"The sands carried by the winds when passing over rocks sometimes
_wear them smooth_, or cover them with _scratches and furrows_, as
observed by W. P. Blake on granite rocks at the Pass of San
Bernardino, in California. Even quartz was polished and garnets were
left projecting upon pedicels of feldspar. Limestone was so much worn
as to look as if the surface had been removed by solution. Similar
effects have been observed by Winchell in the Grand Traverse region,
Michigan. Glass in the windows of houses on Cape Cod sometimes has
holes worn through it by the same means. The hint from nature has led
to the use of sand, driven by a blast, with or without steam, for
cutting and engraving glass, and even for cutting and carving granite
and other hard rocks."[1]

Gratacap describes the rock underneath the "till" as polished and
oftentimes lustrous."[2]

But, it may be said, if it be true that _débris_, driven by a