12 THE DEPOSITS OF THE SEA-BOTTOM. 



According to this table it seems, as if the maximum is found between the depths of 1500 and 

 2000 faths, that is to say, just at the largest depths reached by the Iugolf Expedition; the large 

 percentages, given from the smaller depths, must be taken to have originated in casual circumstances; 

 Murray, it is true, draws himself from these large percentages the inference that the amount of 

 carbonic acid generally decreases with increasing depth, and adds moreover that this fact would be 

 more strikingly exhibited, if only the circumstances of each single region were taken into considera- 

 tion. It seems, however, to be evident that if we had a continuous series of specimens from a single 

 region, we should find, that the Globigerina clay, which always towards the land begins with 

 30 per ct CaC0 3 , would, as to the carbonate of lime, increase quite evenly to the greatest percent- 

 age it should reach in the place in question, and then decrease as evenly, until by and by it merged 

 into the Red deep-sea clay, in which the carbonate of lime is often completely wanting. 



If a comparison be made between the percentages of carbonate of lime from the two expedi- 

 tions, it will be seen that on corresponding depths it is considerably larger in the specimens of the 

 Challenger Expedition. From this may be drawn the conclusion that in the colder seas the circum- 

 stances are not so favourable for the formation of lime-charged Globigerina clay as in the warmer 

 seas, the same result we also arrived at by considering the distance from land, in which the Globi- 

 gerina clay was found; what these circumstances are will be examined later. 



The relation between tbe Globigerina clay and the common chalk has often before been 

 discussed 1 ), and there is no special cause for treating this question here. The forms of Globigerina 

 clay brought home by the present expedition, are rather different from the chalk as to their amount 

 of lime, and accordingly other conditions must have been predominant by the formation of this latter. 

 If we go to the warmer seas we shall find, according to Murray, that the largest amount of carbonate 

 of lime found in Globigerina clay, is 96-80 per ct, near Ascension, while the chalk often contains 

 upwards of 99 per ct of carbonate of lime. The difference, to be sure, is not so large, if we con- 

 sider that the chalk contains strata of flint that must have been formed by silica originating by the 

 dissolution of siliceous organisms originally evenly distributed throughout the whole mass; but if 

 we regard the number of mineral grains we shall find a far greater difference between the two rocks. 

 The least amount of sandy ingredients I have found in Globigerina clay, is o-86 per ct. in the 

 specimen no 80; and even in the most lime-charged specimens from the Challenger-Expedition 1 per 

 ct of mineral grain is noted. But according to Hume the amount of minerals in the chalk may 

 decrease to 0-0026 per ct, i. e. about a 400 times smaller quantity than in the Globigerina clay. 

 This fact is still more remarkable considering the nature of the minerals in both sorts of deposits. 

 In the chalk the most prominent mineral is the quartz, while this mineral is only little conspicuous 

 in the specimens of Globigerina clay taken farthest from land in the oceans, in which specimens the 

 chief part of the sandy ingredients consists of lapilli, pumice, volcanic glass, felspar, augite, olivine a. s. o. 

 Xearer to the land, on the contrary, the quartz is generally prominent in the Globigerina clay. Ac- 

 cordingly we may, in analogy with this, make the conclusion that the chalk must have been formed 

 rather close to the land, but, to be sure, under circumstances found nowhere at the present time. 



«) C Wyville Thomson: The Depths of the Sea. London 1873. 



W. F. Hume: Researches on the upper cretaceous zones of the south of England. London 1893. 



