Ill EXPEDITION OF THE " CHALLENGER " 95 



Professor Thomson gives the following account 

 of this capital discovery : 



" According to our present experience, the deposit of Globi- 

 gerina ooze is limited to water of a certain depth, the extreme 

 limit of the pare characteristic formation being placed at a depth 

 of somewhere about 2, 250 fathoms. Crossing from these shal- 

 lower regions occupied by the ooze into deeper soundings, we 

 find, universally, that the calcareous formation gradually passes 

 into, and is finally replaced by, an extremely tine 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 several hundred fathoms of increasing depth ; the 

 shells gradually lose their sharpness of outline, and assume a 

 kind of * rotten ' look and a brownish colour, 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. This 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 colour of chocolate. 



" In indicating the nature of the bottom on the charts, we 

 came, from experience and without any theoretical considera- 

 tions, to use three terms for soundings in deep water. Two of 

 these, Gl. oz. and r. cl., 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 ' Qlobigerina ooze' or 'red clay,' and before we 

 were fully aware of the nature of these, we were in the habit of 

 indicating them as 'grey ooze ' (gr. oz.) We now recognise the 

 'grey ooze ' as an intermediate stage between the Globigerina 

 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 calcareous ooze, while on the other, the ooze is mixed 

 with an increasing proportion of 'red clay.' 



