PHYSiOGRAPIliCAL GEOLOGY. 183 



and the American Tuscarora, all of which were carried out 

 almost simultaneously in the years 1872-77. These were 

 followed by a series of similar undertakings. 1 



The seas were investigated in all latitudes and in all zones 

 by means of plumb-line soundings and deep-sea thermometer 

 readings ; and the ocean sediments were brought from 

 different horizons of depth by dredging-nets. In the year 

 1843, Humboldt had known no greater depth than 2000 

 metres. From the large number of observations taken by the 

 Challenger and Tuscarora expeditions, Samuel Haughton was 

 able in 1876 to calculate the mean depth of the Pacific, 

 Atlantic, and Indian Oceans at 3000 to 3,650 metres. 

 Kriimmel in 1878 made a most careful and accurate 

 calculation from all known data, and gave the mean depth for 

 all oceans at 3,438 metres. 



The old hypotheses of Athanasius Kircher, Kant, Ritter, 

 and others, about submerged mountain-systems and submarine 

 prolongations of continents had to give place to the newly 

 obtained data. Jt was found that the greatest ocean depths 

 were not in the middle of the oceans, but as a rule along the 

 edge of mountainous coast-lines. The floor of the ocean has 

 its different horizons of level : smooth ridges, extensive 

 plateaux with gentle slopes, narrow canal-like depressions, 

 connected series of deep hollows extending to depths of 6000 

 metres, and even 8,500 metres below sea-level, and undulating 

 crust-forms occur in all the great oceans ; but under the water 

 there are no toothed mountain summits, no steep aretes, no 

 valleys and ravines such as we are familiar with amongst the 

 surface forms of the land produced by subaerial erosion. 



The material brought up by dredging-nets shows the nature 

 of the sediments that are in course of deposition on the ocean- 

 floor. "On the continental shelf, within the roo-fathom line, 

 sands and gravels predominate, while on the continental slopes 

 beyond the loo-fathom line, blue muds, green muds, and red 

 muds, together with volcanic muds and coral muds, prevail, 

 the two latter kinds of deposits being, however, more character- 

 istic of the shallow water around oceanic islands. The com- 

 position of all these terrigenous deposits depends on the 

 structure of the adjoining land. 



1 A complete account of the expeditions which have contributed to our 

 scientific knowledge of oceanography has been given up to the year 1883 in 

 Boguslawsky's Handbuch der Oceanographie^ vol. i., pp. 390-400. 



