348 PHYSIOGRAPHY 



Either the land where such reefs occur has risen greatly, or the 

 sea-level has been depressed. There are very ancient coral reefs 

 (now limestone) in the interior of continents, as, for example, in 

 eastern Wisconsin, showing that a warm sea once covered this 

 region. 



The materials of the sea bottom are gravel, sand, mud, shells, 

 coral, etc., and ooze. Gravel is found chiefly along the borders of 

 the land, out to depths of a few fathoms, or at most a few score of 

 fathoms. Gravel and bowlders, carried out by icebergs, are occa- 

 sionally found at great depths, and far from land. Sand, too, is 

 found chiefly in shallow water, though it extends out to depths 

 greater than those reached by gravel. Mud is much more wide- 

 spread. 



Dredging. The sediment on the bottom of the sea may be 

 brought up to the surface. Various sorts of apparatus are used for 

 this purpose. The cup lead is shown in Fig. 275. B is a hollow 

 inverted cone. Above the cone is a sliding disc, D, a little larger 

 than the base of the cone. This apparatus is let down, and the 

 cone sinks into the soft sediment and is filled with it. On being 

 raised, the disc shuts down on the cup and holds its contents in. 



Fig. 276 shows a dredge. The dredge is let down, and the flar- 

 ing strip of metal E is dragged along the bottom, and turns the 

 sediment of the bottom into the sack. Swabs are attached below 

 to entangle small animals missed by the dredge. 



By the use of dredges it is known that most of the bottom of 

 the sea is covered with soft sediment. This sediment has come 

 from many sources. Some of it was carried to the sea by rivers, 

 some of it was worn from the shores by the waves, some of it was 

 blown from the land, some of it is made up of the shells, etc., of 

 the organisms which live in the water, and some of it is com- 

 posed of fine debris thrown out from volcanoes beneath the sea. 

 A little cosmic ("shooting-star") dust is also present. 



The sediments of the sea bottom are to be looked upon as rock in 

 the making, for all sediments in the sea may become solid rock by 

 being cemented together. This process is now taking place at 

 many points in the bottom of the sea. In some it takes place as 

 fast as the sediments gather* 



