40 THE POPULAR SCIENCE MONTHLY, 



when it reaches 3,150 fathoms; tliere the clay is pure and smooth, and contains- 

 scarcely a trace of lime. From tliis great depth the bottom gradually rises, and, 

 with decreasing depth, the gray color and the calcareous composition of the 

 ooze return. Three soundings, in 2,050, 1,900, and 1,950 fathoms on tlie 'Dol- 

 pliin rise,' gave highly-characteristic examples of tlie Gloligerina 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 i)urity ; and 

 our last sounding, in 1,420 fatlioins, before roacliing Sombrero, restored the 

 Olol)i(ferina ooze with its peculiar associated fauna. 



"This section shows also the wide extension and the vast geological impor- 

 tance of the red-clay formation. The total distance from Teneriffe to Sombrero- 

 is about 2,700 ruiles. Proceeding from east to went we have 



About 80 miles of volcanic mud and sand, 

 " 350 " Gloiigerina ooze, 

 " 1,050 " red clay, 

 " 330 " Glohigerina ooze, 

 " 850 " red clay, 

 " 40 " Gloligerina ooze; 

 giving a total of 1,900 miles of red clay to Y20 miles of Glohigerina 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 tliat it is in tlie 

 main solved, yet some matters of detail are still involved in difficulty. My first 

 impression was that it might be the most minutely-divided material, the ultimate 

 sedhnent produced by the disintegration of the land, by rivers and by the action 

 of the sea on exposed coasts, and held in suspension and distributed by ocean- 

 currents, and only making itself manifest in places unoccupied by the Glolige- 

 rina ooze. Several circumstances seemed, however, to negative this mode of 

 origin. The formation seemed too uniform ; wherever we met with it, it had the 

 same character, and it only varied in composition in containing less or more 

 carbonate of lime. 



"Again, we were gradually becoming more and more convinced that all the 

 important elements of the Glohigerina ooze lived on the surface, and it seemed 

 evident that, so long as the condition on the surface remained the same, no alter- 

 ation of contour at the bottom could possibly prevent its accumulation ; and 

 the surface conditions in the mid-Atlantic were very uniform, a moderate cur- 

 rent of a very equal temperature passing continuously over elevations and depres- 

 sions, and everywhere yielding to the tow-net the ooze-forming Foraminifera in 

 the same proportion. The mid-Atlantic swarms with pelagic Mollusca, and, in 

 moderate depths, the shells of these are constantly mixed with the Glohigerina 

 ooze, sometimes in number sufficient to make up a considerable portion of its. 

 bulk. It is clear that these shells must fall in equal numbers upon the red-clay, 

 but scarcely a trace of one of them is ever brought up by the dredge on the red- 

 clay area. It might be possible to explain the absence of shell-secreting animals- 

 living on the bottom, on the supposition that the nature of the deposit was inju- 

 rious to them ; but then the idea of a current sufficiently strong to sweep them 

 away is negatived by the extreme fineness of the sediment which is being laid 

 down ; the absence of surface-shells appears to be intelligible only on the suppo- 

 sition that they are in some way removed. 



"We conclude, therefore, that the 'red clay' is not an additional substance 

 introduced from without, and occupying certain depressed regions on account 

 of some law regulating its deposition, but that it is produced by the removal, hy 



