256 THE ATLANTIC. [chap. v. 



more stable combinations, through the decomposition of tlie 

 multitudes of organized beino;s which swarm in the successive 

 layers of the sea; and I am still inclined to refer to this source 

 a great part of the molecular matter which always forms a con- 

 siderable part of a red-clay microscopic preparation. 



There is great difficulty in pointing out rocks belonging to 

 any of the past geological periods which correspond entirely, 

 whether in chemical composition or in structure, with the beds 

 now in process of formation at the bottom of the ocean. There 

 seems every reason to believe that the rocks of the Mesozoic 

 and Cenozoic series, at all events, were formed in comparative- 

 ly .shallow water, and after the prominent features at present 

 existing had been stamped upon the contour of the earth's 

 crust ; and, consequently, that none of these have the essential 

 characters of deep-sea deposits. I imagine, however, that the 

 limestone which would be the result of the elevation and slight 

 metamorphosis of a mass of globigerina ooze would resemble 

 very closely a bed of gray chalk ; and that an enormous accu- 

 mulation of red clay might in time, under similar circumstances, 

 come to be very like one of the Paleozoic schists, such, for ex- 

 ample, as the Cambrian schist with Oldhamia and worm-tracks 

 at Bray Head. It is a very difficult question, however, and one 

 on which I shall offer no opinion until we have very much more 

 complete data from comparative microscopical examination and 

 chemical analysis. 



The Distribution of Ocean Temperature. — Throughout the 

 whole of the Atlantic the water is warmest at the surface. 

 From the surface it cools rapidly for the first hundred fathoms 

 or so ; it then cools more slowly down to live or six hundred 

 fathoms, and then extremely slowly, either to the bottom or to 

 a certain point, from which it maintains a uniform or nearly 

 uniform temperature to the bottom. 



A glance at a series of temperature sections such as those rep- 

 resented in Plates Y., IX., XYL, XX., XXIL, and XXYIII., 



