ghing in all about four ounces, I have detected the broken
remains of no fewer than _sixteen_ hinge joints. And on the same
principle through which the stronger fragments of Cyprina were preserved
in so much larger proportion than the weaker ones, may Cyprina
itself have been preserved in much larger proportion than its
more fragile neighbors. Occasionally, however,--escaped, as if by
accident,--characteristic fragments are found of shells by no means
very strong,--such as _Mytilus_, _Tellina_, and _Astarte_. Among the
univalves I can distinguish _Dentalium entale_, _Purpura lapillus_,
_Turritella terebra_, and _Littorina littorea_, all existing shells, but
all common also to at least the later deposits of the Crag. And among
the bivalves Mr. Dick enumerates,--besides the prevailing _Cyprina
islandica_,--_Venus casina_, _Cardium edule_, _Cardium echinatum_,
_Mytilus edule_, _Astarte danmoniensis_ (_sulcata_), and _Astarte
compressa_, with a _Mactra_, _Artemis_, and _Tellina_.[15] All the
determined species here, with the exception of _Mytilus edule_, have,
with many others, been found by the Rev. Mr. Cumming in the
boulder-clays of the Isle of Man; and all of them are living shells at
the present day on our Scottish coasts. It seems scarce possible to fix
the age of a deposit so broken in its organisms, on the principle that
would first seek to determine its per centage of extinct shells as the
data on which to found. One has to search sedulously and long ere a
fragment turns up sufficiently entire for the purpose of specific
identification, even when it belongs to a well-known living shell; and
did the clay contain some six or eight per cent. of the extinct in a
similarly broken condition (and there is no evidence that it contains a
single per cent. of extinct shells), I know not how, in the
circumstances, the fact could ever be determined. A lifetime might be
devoted to the task of fixing their real proportion, and yet be devoted
to it in vain. All that at present can be said is, that, judging from
what appears, the boulder-clays of Caithness, and with them the
boulder-clays of Scotland generally, and of the Isle of Man,--for they
are all palpably connected with the same iceberg phenomena, and occur
along the same zone in reference to the sea-level,--were formed during
the _existing_ geological epoch.

These details may appear tediously minute; but let the reader mark how
very much they involve. The occurrence of rece