SUBTERRANEAN WATERS. 663 



Stratification is only sparingly produced by a settling of sediment 

 in any one spot in the order of coarseness or density of particles ; for 

 there are usually currents that produce a down-stream assorting. The 

 finest of earthy material is often held long suspended, and it may be 

 slowly deposited over flood-made beds, and top them with a thin layer. 



(c.) Sandbars ; obliquely laminated Structure. — The sandbars of 

 a river channel, as shown by General Warren, have usually a slight 

 pitch up stream and a steep one at the down stream extremity. The 

 sand is carried on until the crest is reached, when it falls over and 

 stops in the still water below. The stratification will correspond with 

 the surface; and as the sand-bar extends itself down stream by the 

 additions to its extremity, the pitch of the down-stream extremity 

 will determine oblique bedding parallel with it. 



The pushing of detritus along the bottom of a river must result in 

 similar oblique bedding. But in both cases, oblique deposition will be 

 followed by deposition in horizontal beds when the floods are declin- 

 ing, so that combinations of the two, often of a very irregular charac- 

 ter, should exist in such deposits. A ripple mark has the same form 

 as the sandbar and for essentially the same reason. 



B. SUBTERRANEAN WATERS. 

 1. The Source and Condition of Subterranean Waters. 



A large part of the water which reaches the earth's surface descends 

 into the soil, and becomes subterranean. 



It mostly passes downward until it reaches a compact layer of some 

 kind, — as of clay, or agglutinated pebbles ("hard-pan"), or of hard 

 rock, — and upon this it may accumulate largely during rainy seasons. 

 And if the layer has a regular pitch in any direction, these waters will 

 flow along the sloping surface, and often gather from different direc- 

 tions around into streams. They may descend ,. between the inclined 

 beds of a region, and continue on the descent for hundreds and even 

 thousands of feet in depth. 



A coral island but ten feet high and a few hundred yards wide, and 

 consisting of coral rock up to the water-level with coral sands above, 

 generally yields, from excavations to the surface of the rock beneath, 

 a sufficient supply of water for its inhabitants, and all of it has come 

 from the rains. The fresh water, moreover, is sufficient to exclude, 

 by its seaward pressure, all ingress of salt water. If this is true on a 

 coral island, the subterranean waters derived from the rains over large 

 hilly islands or lands should be very great. The proportion which 

 becomes subterranean depends on the permeability or cavernous na- 

 ture of the subjacent rocks or material. Crystalline rocks absorb al- 



