"The depth goes on increasing to a distance of 1,150 miles from
Teneriffe, when it reaches 3,150 fathoms; there the clay is pure and
smooth, and contains scarcely a trace of lime. From this great depth the
bottom gradually rises, and, with decreasing depth, the grey colour and
the calcareous composition of the ooze return. Three soundings in 2,050,
1,900, and 1,950 fathoms on the 'Dolphin Rise' gave highly characteristic
examples of the _Globigerina_ formation. Passing from the middle plateau
of the Atlantic into the western trough, with depths a little over 3,000
fathoms, the red clay returned in all its purity; and our last sounding,
in 1,420 fathoms, before reaching Sombrero, restored the _Globigerina_
ooze with its peculiar associated fauna.

"This section shows also the wide extension and the vast geological
importance of the red clay formation. The total distance from Teneriffe
to Sombrero is about 2,700 miles. Proceeding from east to west, we have--

About 80 miles of volcanic mud and sand,
" 350 " _Globigerina_ ooze,
" 1,050 " red clay,
" 330 " _Globigerina_ ooze,
" 850 " red clay,
" 40 " _Globigerina_ ooze;

giving a total of 1,900 miles of red clay to 720 miles of _Globigerina_
ooze.

"The nature and origin of this vast deposit of clay is a question of the
very greatest interest; and although I think there can be no doubt that
it is in the main solved, yet some matters of detail are still involved