RHIZOPODA. 21 



matter held in suspension in the searwater, which slowly sinks to the undisturbed 

 depths. 



Nevertheless, the deepest ocean-floor is not always devoid of organic remains. The 

 deepest sounding by the " Challenger " was made in the Pacific Ocean on the 23d of 

 March, 1875, and showed a depth of four thousand five hundred and seventy-five 

 fathoms. The bottom was covered with what resembled the ordinary red clay to the 

 eye, but it was gritty, and contained such a large proportion of radiolaiian remains 

 that it received the name of Radiolarian ooze. It had previously been supposed that 

 even ^these silioious remains, together with the frustules of diatoms, which are more or 

 less abundant in the Globigerina ooze, disappeared with the calcareous shells, in some 

 manner not fully understood. The occurrence of the deep Radiolarian ooze, however, 

 has shown that there is no destruction of silicious shells, and their accumulation in so 

 much greater abundance there than in the Globigerina ooze was accounted for by Prof. 

 "Wyville Thomson on the supposition, based upon the results of collections at different 

 depths down to one thousand fathoms with the tow-net, — that Radiolaria lived at aU 

 depths, and therefore, where the water was deepest the accumulations of their skele- 

 tons would be the most rapid. Later investigations by Mr. Agassiz, using an ingenious 

 apparatus devised by Lieut.-Commander Sigsbee, IT. S. N"., have shown that there is 

 no life between a narrow zone where the surface animals are found and the habitat of 

 those living on or very near the bottom. 



The deepest cast that has ever been made was made from the TJ. S. Coast Survey 

 Steamer "Blake" in January, 1883, in latitude 19° 39' 10-/ N., and longitude 66° 26/ 

 05" W., between the Bermudas and the Bahamas, about one hundred miles N. W. of 

 St. Thomas. The depth there found was four thousand five hundred and sixty-one 

 fathoms ; the temperature of the deepest water was 36° F. Another cast in latitude 19° 

 29' 30" N., longitude 66° 11' 45", showed a depth of four thousand two hundred and 

 twenty-three fathoms. At these great depths, — more than five statute miles beneath 

 the surface, a depth equal to the height of the highest mountains in the world — the bot- 

 tom is covered with a very fine brown ooze, containing a few Diatoms and sponge 

 spicules. 



In the North Pacific, at depths of three thousand and four thousand fathoms, are 

 found tests of Trochammina (Ammodiscus) incerta, one of the arenaceous forms. At 

 great depths are also found species of Miliola, usually more or less incrusted with 

 grains of sand, while in some cases the shell consists not of lime, but of clear, homo- 

 geneous silica which wUl resist the action of acids like the frustules of diatoms. 



Some of the genera of Foraminif era have had a great range in geologic time. In 

 the lower Silurian rocks of Russia — in the so-called Ungulite grit — Ehrenberg found 

 green sand casts of genera now living, Textularia, Gattulina, and Rotalia. The 

 oldest form of life of which the rocky strata furnish any remains is possibly the Eozoon 

 Canadense, found preserved in greatest perfection in the Laurentian rocks of Canada. 

 Concerning the nature of Eozoon — the dawn-animal — there has been much contro- 

 versy. On the one hand it is claimed with great probability that the peculiar struc- 

 tures found in the rocks are of purely mineral origin. But Dr. J. W. Dawson and 

 Dr. William B. Carpenter, who have studied this subject with great care, have 

 declared that the structure of Eozoon corresponds in every particular with that of cer- 

 tain Foraminifera of the vitreous type. The successive chambers connected by pas- 

 sages, the intermediate skeleton with its complex system of inosculating canals, the 

 minutely tubular " nummuline layer," have all been claimed to have been found and 



