ca 
436 ; DYNAMICAL GEOLOGY. —— [Boox IIT. — 
and streams from the land. Dr. Carpenter found the bottom — 
waters of the Mediterranean to be everywhere permeated by an — 
extremely fine mud, derived no doubt from the rivers and shores of 
that sea. He remarks that the characteristic blueness of the 
Mediterranean, like that of the Lake of Geneva, may be due to 
the diffusion of exceedingly minute sedimentary particles through 
the water. 
During the voyage of the Challenger, from the abysses of the 
Pacific Ocean, at remote distances from land, the dredge brought up 
bushels of rounded pieces of pumice of all sizes up to blocks a foot in 
diameter. These fragments were all evidently water-worn, as if 
derived from land, though we are still ignorant of the extent to 
which they may have been supplied by submarine volcanic eruptions. 
Some small pieces were taken on the surface of the ocean in the 
tow-net. Round volcanic islands, and off the coasts of volcanic tracts 
of the mainland, the sea is sometimes covered with floating pieces of | 
water-worn pumice swept out by flooded rivers. These fragments © 
may drift away for hundreds or even thousands of miles until, 
_ becoming water-logged, they sink to the bottom. The universal 
distribution of pumice was one of the most noticeable features in 
the dredgings of the Challenger. The clay which is found on the 
bottom of the ocean at the greatest distances from any shore 
contains only volcanic minerals and appears to be due to the tritura- 
tion of volcanie detritus. At a distance of several hundred miles 
from shore traces of the minerals of the crystalline rocks of the land 
begin to make their appearance,} 
Another not unimpurtant process of marine transport is that 
performed by floating ice. Among the Arctic glaciers moraine stuff 
is of rare occurrence ; but occasional blocks of rock and heaps of 
earth and stones fall from the cliffs which rise above the general waste 
of snow. Hence on the icebergs that float off from these glaciers, 
rock débris may sometimes be observed. It is transported southward 
for hundreds of miles until, by the shifting or melting of the bergs, — 
it is dropped into deep water. The floor of certain portions of the 
North Atlantic in the pathway of the bergs may be plentifully 
strewn with this kind of detritus. By means of the ice-foot also, an 
enormous quantity of earth and stones is every year borne away from 
the shore by the disrupted ice, and is strewn over the floor of the 
sounds, bays, and channels. 
(4.) Reproduction. —The sea, being the receptacle for the 
material worn away from the land, must receive and store up in its 
depths all that vast amount of detritus by the removal of which the 
level and contours of the land are in the course of time so greatly 
changed, ‘The deposits which take place within the area covered by 
the sea may be divided into two groups—the inorganic and organic. 
It is the former with which we have at present to deal; the latter 
will be discussed with the other geological functions of plants and 
* Murray, Proc. Roy. Soc, Edin. 1876-7, p. 247. 


