372 PEECT SLA DEN TEUST EXPEDITION. 



deposits is examined from the same tropical area but from different depths, it has been 

 found that in depths of between say 500 and 1000 fathoms every species of calcium 

 carbonate shell found in the tow-nets at the surface can be detected in the deposit at the 

 bottom. 



In greater depths — say between 1000 and 2000 fathoms — all the thinner and more 

 delicate shells have disappeared from the deposit, especially the Pteropod, Heteropod, 

 and the smaller and more delicate of the pelagic Poraminifera shells. 



In still greater depths only the heavier and more massive shells of Glohigerina, 

 Fullenia, Sphceroidina, and Pidvinulina are present in the deposit, and many of these 

 are holed and have a weathered appearance. In the greatest depths, between 3000 and 

 5000 fathoms, it is often difficult to find even a trace of these pelagic shells. 



There seems to be no doubt about these facts, although different views may be held as 

 to the cause of the disappearance of the shells with increasing depth. My own view is 

 that solution of dead calcium carbonate shells and skeletons is everywhere taking place 

 in the ocean, except where the water is completely saturated with calcium carbonate. 

 As soon as a lime-secreting organism dies at the surface of the ocean it commences to 

 fall towards the bottom, and its shell is exposed to solution from the action of sea-water 

 and carbonic acid, the latter possibly being produced by the decomposition of its own 

 body. The great majority of the shells are, however, only partially dissolved during 

 the fall through the first few hundred fathoms, and on reaching the bottom at these 

 lesser depths they accumulate there, being soon covered up by the fall of other shells. 

 Sea-water can only take up a relatively small quantity of calcium carbonate before being 

 saturated. Tlie water mixed up with, and in immediate contact with, the deposit would 

 soon be in this condition, and the process of solution would be wholly or partially 

 arrested, so that the shells would accumulate on the bottom. In the greater depths 

 many of the thinner shells would be wholly dissolved before reaching the bottom; those 

 that did reach it would not be covered up so quickly as in shallow depths, and would be 

 longer exposed to water still capable of taking up calcium carbonate in solution. In 

 the greatest depths the shells are almost certainly dissolved before, or soon after, reaching 

 the bottom. 



Two instances of the relation between the percentage of calcium carbonate and depth 

 may be cited from the deposits collected by H.M.S. ' Egeria ' in the Indian Ocean in 

 1887. Thus in lat. 20° 42' S., long. 73° 10' E., depth 2312 fathoms, the deposit was 

 a Glohigerina Ooze containing 73'3S per cent, of calcium carbonate, while at the 

 neighbouring station in lat. 27° 7' S., long. 77° 49' E., depth 2564, the deposit was a 

 Red Clay containing no calcium carbonate. Again, in lat. 29° 56' S., long. 54° 6' E., 

 depth 2312 fathoms, the deposit was again a Glohigerina Ooze containing 69'95 per cent, 

 of calcium carbonate, while in lat. 23° 15' S., long. 56° 18' E., depth 2466 fathoms, 

 the deposit was a Hed Clay containing 1222 per cent, of calcium carbonate, and in 

 lat. 26° 23' S., long. 55° 25' E., depth 2876 fathoms, the deposit was also a Red Clay 

 containing no calcium carbonate. 



When the calcium carbonate is removed from a typical sample of Glohigerina Ooze 



