-slips, to facilitate the progress of his vessel seawards,
is not more treacherous to the tread: while the Upper Liasic deposit
which rests over it is composed of a dark slaty shale, largely charged
with bitumen. And of a Liasic deposit of this compound character,
consisting in larger part of an inferior argillaceous bed, and in lesser
part of a superior one of dark shale, the tile-clay of Blackpots has
been formed.

I had next to determine whether aught remained to indicate the period of
its re-formation. The tile-works at the point of the promontory rest on
a bed of shell-sand, composed exclusively, like the sand so abundant on
the western coast of Scotland, of fragments of existing shells. These,
however, are so fresh and firm, that, though the stratum which they form
seems to underlie the clay at its edges, I cannot regard them as older
than the most modern of our ancient sea-margins. They formed, in all
probability, in the days of the old coast line, a white shelly beach,
under such a precipitous front of the dark clay as argillaceous deposits
almost always present to the undermining wear of the waves. On the
recession of the sea, however, to its present line, the abrupt, steep
front, loosened by the frosts and washed by the rains, would of course
gradually moulder down over them into a slope; and there would thus be
communicated to the shelly stratum, at least at its edges, an underlying
character. The true period of the re-formation of the deposit was, I can
have no doubt, that of the boulder-clay. I observed that the septaria
and larger masses of shale which the bed contains, bear, on
roughly-polished surfaces, in the line of their larger axes, the
mysterious groovings and scratchings of this period,--marks which I have
never yet known to fail in their chronological evidence. It may be
mentioned, too, simply as a fact, though one of less value than the
other, that the deposit occurs in its larger development exactly where,
in the average, the boulder-clays also are most largely developed,--a
little over that line where the waves for so many ages charged against
the coast, ere the last upheaval of the land or the recession of the sea
sent them back to their present margin. There had probably existed to
the west or north-west of the deposit, perhaps in the middle of the open
bay formed by the promontory on which it rests,--for the small
proportion of other than Liasic materials which it contains serves to
show tha