214 THE ATLANTIC. [cHapP. Il. 
the Atlantic into the western trough, with depths a little over 
3000 fathoms, the red clay returned in all its purity; and our 
last sounding, in 1420 fathoms, before reaching Sombrero, re- 
stored the globigerina ooze with its peculiar associated fauna. 
This section shows also the wide extension and the vast geo- 
logical importance of the red-clay formation. The total dis- 
tance from Teneriffe to Sombrero is about 2700 miles. Pro- 
ceeding from east to west, we have 
About 80 miles of volcanic mud and sand, 
esp Or ae globigerina ooze, 
“NOb0 Ne red clay, 
AE Ae globigerina ooze, 
SE tet0) red clay, 
so A cs globigerina ooze, 
giving a total of 1900 miles of red clay to 720 miles of globige- 
rina 00ze. 
The following table, taken.from the chart, gives a good gen- 
eral idea of the distribution of the two formations with regard 
to depth; it being understood, however, that while in all the 
soundings marked “red clay” the characters of that formation 
greatly predominated, in several of the more shallow of these 
the change was by no means complete. The table gives an 
average of 1600 fathoms for our soundings in this section in 
the globigerina ooze: this is a datum of no value, for we sound- 
ed only once in shallow water (450 fathoms), and we know that 
this formation covers large areas at depths between 300 and 
400 fathoms; but the mean maximum depth at which it oc- 
curs is important, and may be taken at about 2250 fathoms. 
The mean depth of the red-clay soundings is about 2750 fath- 
oms. The general concurrence of many observations would go 
far to prove, what seems now, indeed, to stand in the position 
of an ascertained fact, that wherever the depth increases from 
about 2200 to 2600 fathoms, the modern chalk formation of the 
Atlantic passes into a clay. 
