EXPEDITION OF THE CHALLENGER. 39 



of the organisms Avhich undoubtedly abound at tlie surface of tlie 

 3Iediterranean ; and it would seem to have no application to the 

 remarkable fact discovered by the Challenger, that in the open Atlan- 

 tic and Pacific Oceans, in the midst of the great intermediate zone, 

 and tliousands of miles away from the embouchure of any river, the 

 sea-bottom, at depths approaching to and beyond 3,000 fathoms, no 

 longer consists of Glohlgerina ooze, but an excessively fine red clay. 



Prof. Thomson gives the following account of this capital dis- 

 covery : 



"According to oiu- present experience, the deposit of Glohigerina ooze is 

 limited to water of a certain depth, the extreme limit of the pure characteristic 

 formation being placed at a depth of somewhere about 2,250 fathoms. Crossing 

 from these shallower regions occupied by the ooze into deeper surroundings, we 

 find universally that the calcareous formation gradually passes into, and is finally 

 replaced by, an extremely fine pure clay, which occupies, speaking generally, all 

 depths below 2,500 fathoms, and consists almost entirely of a silicate of the red 

 oxide of iron and alumina. The transition is very slow, and extends over sev- 

 eral hundred fathoms of increasing depth ; the shells gradually lose their sharp- 

 ness of outline, and assume a kind of 'rotten' look and a brownish color, and 

 become more and more mixed with a fine amorphous red-brown powder, which 

 increases steadily in proportion, until the lime has almost entirely disappeared. 

 The brown matter is in the finest possible state of subdivision ; so fine that, 

 when, after sifting it to separate any organisms it might contain, we put it into 

 jars to settle, it remained for days in suspension, giving the water very much 

 the appearance and color of chocolate. 



" In indicating the nature of the bottom on the charts, we came, from expe- 

 rience and without any theoretical considerations, to use three terms for sound- 

 ings in deep water. Two of these, Gloligerina ooze and red clay, were very 

 definite, and indicated strongly-marked formations, with apparently but few 

 characters in common; but we frequently got soundings which we could not 

 exactly call ^ Gloiigerina oozq^ or 'red clay,' and, before we were fully aware 

 of the nature of these, we Avere in the habit of indicating them as ' gray ooze ' 

 (gr. oz.). We now recognize the 'gray ooze' as an intermediate stage between 

 the Gloligerina ooze and the red clay; we find that on one side, as it were, of 

 an ideal line, the red clay contains more and more of the material of the cal- 

 careous ooze, while on the other the ooze is mixed with an increasing propor- 

 tion of ' red clay.' 



"Although we have met with the same phenomenon so frequently that we 

 were at length able to predict the nature of the bottom from the depth of the 

 surroundings with absolute certainty for the Atlantic and the Southern Sea, we 

 had, perhaps, the best opportunity of observing it in our first section across the 

 Atlantic, between Tenerifte and St. Thomas. The first four stations on this sec- 

 tion, at depths from 1,525 to 2,220 fathoms, show Gloligerina ooze. From the 

 last of these, which is about 300 miles from Tenerifi'e, the depth gradually 

 increases to 2,740 fathoms at 500, and 2,950 fathoms at 750 miles from Tenerifi'e. 

 The bottom in these two soundings might have been called 'gray ooze,' for, 

 although its nature has altered entirely from the Gloligerina ooze, the red clay 

 into which it is rapidly passing still contains a considerable admixture of car- 

 bonate of lime. 

 , "The depth goes on increasing to a distance of 1,150 miles from Tenerifi'e, 



