Oldhamia, silicious sponges, and thin-shelled peculiar shrimps. Such
formations, more or less metamorphosed, are very familiar, especially to
the student of palaeozoic geology, and they often attain a vast thickness.
One is inclined, from the great resemblance between them in composition
and in the general character of the included fauna, to suspect that these
may be organic formations, like the modern red clay of the Atlantic and
Southern Sea, accumulations of the insoluble ashes of shelled creatures.

"The dredging in the red clay on the 13th of March was usually rich. The
bag contained examples, those with calcareous shells rather stunted, of
most of the characteristic deep-water groups of the Southern Sea,
including _Umbellularia, Euplectella, Pterocrinus, Brisinga, Ophioglypha,
Pourtalesia_, and one or two _Mollusca_. This is, however, very rarely
the case. Generally the red clay is barren, or contains only a very small
number of forms."

It must be admitted that it is very difficult, at present, to frame any
satisfactory explanation of the mode of origin of this singular deposit
of red clay.

I cannot say that the theory put forward tentatively, and with much
reservation by Professor Thomson, that the calcareous matter is dissolved
out by the relatively fresh water of the deep currents from the Antarctic
regions, appears satisfactory to me. Nor do I see my way to the
acceptance of the suggestion of Dr. Carpenter, that the red clay is the
result of the decomposition of previously-formed greensand. At present
there is no evidence that greensand casts are ever formed at great
depths; nor has it been proved that _Glauconite_ is decomposable by the
agency of water and carbonic acid.