12 GEOLOGY, 



Sea and the Hudson Bay, may be called epicontinental seas, for they 

 really lie upon the continent, or at least upon the continental platform ; 

 while those other detached bodies of water which occupy deep depres- 

 sions in the surface are to be regarded as true abysmal seas, as, for 

 example, the Mediterranean and Caribbean seas and the Gulf of Mexico, 

 whose bottoms are as profound as many parts of the true ocean basin 

 itself. 



Diversities of surface. — The bottoms of the oceanic basins are 

 diversified by broad undulations which range through many thousands 

 of feet, but they are not carved into the diversified forms that give 

 variety to land surfaces. The ocean bottoms are also diversified by 

 volcanic peaks, many of which rise to the surface and constitute isolated 

 islands. Some of them have notable platforms at or near the surface, 

 cut by the waves or built up by the accumulation of sediment and of 

 coralline and other growths about them. Aside from these encircling 

 platforms, the solid surface usually shelves rapidly down to abysmal 

 depths, so that the islands constitute peaks whose heights and slopes 

 would seem extraordinary if the ocean were removed. 



The surface of the land is diversified in a similar way by broad 

 undulations and volcanic peaks, and also by narrower wrinklings and 

 foldings of the crust ; but all of these irregularities have been carved 

 into diversified and picturesque forms by subaerial erosion. In this 

 respect the surface of the land differs radically from the bed of the 

 sea. The agencies which have produced the continental platforms 

 and abysmal basins, and the great undulations and foldings, as well 

 as the volcanic extrusions that mark them, are yet subjects of debate. 

 Here lie some of the most difficult problems of geology, but these cannot 

 be stated with sufficient brevity to find a place here. 



The surface mantle of the lithosphere.^ — The surface of the litho- 

 sphere is very generally mantled by a layer of loose material composed 

 of soil, clay, sand, gravel, and broken rock. This loose material is 

 sometimes known as mantle rock, and sometimes as rock waste. On 

 the land, mantle rock is often composed of the disintegrated products 

 of underlying rock formations. It represents the results of the recent 

 action of the atmosphere, of water, of changes of temperature, and 

 of other physical agencies acting on the outer part of the rock 

 sphere. The surface of this mantle is being constantly rem.oved 

 by wind and water, but as constantly renewed by continued decomposi- 

 tion of the rock below. In some areas, especially in the northern part 



