_Glauconite_ does not take place at great depths, but rather in what may
be termed a sublittoral region, ranging from a hundred to three hundred
fathoms. It cannot be ascribed to any local cause, for it takes place,
not only over large areas in the Gulf of Mexico and the Coast of Florida,
but in the South Atlantic and in the Pacific. But what are the conditions
which determine its occurrence, and whence the silex, the iron, and the
alumina (with perhaps potash and some other ingredients in small
quantity) of which the _Glauconite_ is composed, proceed, is a point on
which no light has yet been thrown. For the present we must be content
with the fact that, in certain areas of the "intermediate zone,"
greensand is replacing and representing the primitively calcareo-
silicious ooze.

The investigation of the deposits which are now being formed in the basin
of the Mediterranean, by the late Professor Edward Forbes, by Professor
Williamson and more recently by Dr. Carpenter, and a comparison of the
results thus obtained with what is known of the surface fauna, have
brought to light the remarkable fact, that while the surface and the
shallows abound with _Foraminifera_ and other calcareous shelled
organisms, the indications of life become scanty at depths beyond 500 or
600 fathoms, while almost all traces of it disappear at greater depths,
and at 1,000 to 2,000 fathoms the bottom is covered with a fine clay.

Dr. Carpenter has discussed the significance of this remarkable fact, and
he is disposed to attribute the absence of life at great depths, partly
to the absence of any circulation of the water of the Mediterranean at
such depths, and partly to the exhaustion of the oxygen of the water by
the organic matter contained in the fine clay, which he conceives to be
formed by the finest particles of the mud brought down by the rivers
which flow into the Mediterranean.