EEPORT ON THE DEEP-SEA DEPOSITS. 
227 
These analyses represent the composition of the three deposits as almost identical. 
In the first place, they are specially characterised by the high percentage of carbonate of 
lime, much higher than the average percentage in a Globigerina Ooze, which in this case 
must be attributed to the presence of Pteropod, Heteropod, and other delicate and larger 
shells, which are absent or rare in the last-named deposit. Having regard to the small 
quantities of alumina, silica, and iron, there can be but little argillaceous and ferruginous 
matters present, and the loss on ignition can most probably be referred to the organic 
matter associated with the calcareous shells. There is nothing special to remark regard- 
ing the sulphate and phosphate of lime, and carbonate of magnesia. These have, it is 
evident, the same origin as in the case of the Globigerina Oozes; it may be noted, how- 
ever, that the magnesia does not augment with the proportionally great increase of 
carbonate of lime.^ 
As the soluble portion rises to a mean of more than 93 per cent, of the whole 
deposit, few anhydrous minerals could be expected in the insoluble portion, and indeed 
no quantitative analysis has been attempted of the 3 per cent, of which it is made up. 
With reference to the Pteropod Ooze, it may be here stated that with the view of 
determining whether or not these shells contained mineral matters other than carbonate of 
lime, an analysis was made of a number of the shells of Cavolinia taken at the surface of 
the sea and still containing the animal. After having removed with all possible care the 
whole of the animal matter, these shells were analysed, and we were able to detect, in 
addition to the carbonate of lime, only traces of iron and of organic matter. 
From all the foregoing considerations, then, we arrive at the conclusion that a Pteropod 
Ooze differs from a Globigerina Ooze only in the larger percentage of carbonate of lime, 
and it has already been pointed out that this is due to the greater abundance of the more 
delicate shells of pelagic organisms. 
Pteropod Ooze was found by the Challenger Expedition only in the Atlantic Ocean. 
It was met with in its most typical form on the central ridges of the Atlantic, where the 
depths did not exceed 1400 fathoms; in these regions it is estimated to cover about 400,000 
square miles of the sea-floor. Had the Challenger been fortunate enough to discover 
similar ridges far from land in the tropical and subtropical regions of the Pacific, they 
would in all probability have been found to be covered by Pteropod Ooze. In nearly all 
cases these shells are very numerous in the shallower depths near tropical lands, but 
usually they are not in sufficient abundance to give a distinctive character to the deposit, 
being masked by the large quantity of other more rapidly accumulating materials either 
of an organic or inorganic nature. In some exceptional cases, however, as off coral reefs 
and oceanic islands, they are sufficiently abundant to allow of the deposit being called a 
Pteropod Ooze, for instance, off the Antilles and the Azores in the Atlantic, and off some of 
1 See pp. 200, 201. 
