we find the rocks covered up by continuous beds of
the boulder-clay, varying generally from twenty to a hundred feet in
thickness, they are, notwithstanding, as completely dressed under the
clay as on the heights above. Had it been a rising land that was
subjected to the attrition of the icebergs, the debris and dressings of
the higher rocks would have protected the lower from the attrition; and
so the thick accumulation of boulder-clay which overlies the old coast
line, for instance, would have rested, not on dressed, but on undressed
surfaces. The barer rocks of the lower levels might of course exhibit
their scratchings and polishings, like those of the higher; but wherever
these scratchings and polishings occurred in the inferior zones, no
thick protecting stratum of boulder-clay would be found overlying them;
and, _vice versa_, wherever in these zones there occurred thick beds of
boulder-clay, there would be detected on the rock beneath no scratchings
and polishings. In order to _dress_ the entire surface of a country from
the sea-line and under it to the tops of its hills, and at the same time
to cover up extensive portions of its low-lying rocks with vast deposits
of clay, it seems a necessary condition of the process that it should be
carried on piece-meal from the lower level upwards,--not from the higher
downwards.

It interested me much to find, that while from one set of appearances I
had been inferring the gradual subsidence of the land during the period
of the boulder-clay, the Rev. Mr. Cumming of King William's College had
arrived, from the consideration of quite a different class of phenomena,
at a similar conclusion. "It appears to me highly probable," I find him
remarking, in his lately published "Isle of man," "that at the
commencement of the boulder period there was a gradual sinking of this
area [that of the island]. Successively, therefore, the points at
different degrees of elevation were brought within the influence of the
sea, and exposed to the rake of the tides, charged with masses of ice
which had been floated off from the surrounding shores, and bearing on
their under surfaces, mud, gravel, and fragments of hard rock." Mr.
Cumming goes on to describe, in his volume, some curious appearances,
which seem to bear direct on this point, in connection with a boss of a
peculiarly-compounded granite, which occurs in the southern part of the
island, about seven hundred feet over the level of the sea.