464 POPULAR SCIENCE MONTHLY. 



The sand grains of which the deposit is largely composed are seen 

 to be arranged in layers nearly horizontal, and these layers are found 

 to be due to alternations of sediment varying in fineness. This 

 phenomenon is called stratification, and is what we should expect of 

 the action of gravity operating on material of different sizes and 

 densities suspended in a body of water. It has been found inexpedi- 

 ent to attempt to show a photograph of this section, owing to the 

 smallness of the subject, but the same phenomena may be observed 

 on a much larger scale in Fig. 5, which will be described below. 



A few rods away the stream that feeds the pool has its origin. 

 The sediment carried by the water and going to build up its delta 

 has its source in part in a neighboring bank made up of material 

 derived from solid rock by weathering, similar to that shown on our 

 first excursion, and partly from older water deposits. Steep channels 

 exist in the disintegrated rock, which represent the material removed 

 by the fast-flowing rain water. 



Now what geological phenomena have we observed at this local- 

 ity? In the first place, it has become clear that running water pos- 

 sesses the power of transporting sediment. In the second place, this 

 sediment has been deposited wherever the velocity of the water has 

 been materially checked. The sediment has been laid down in hori- 

 zontal layers under the influence of gravity. Furthermore, the ma- 

 terial of which the delta is composed has been shown, in part at least, 

 to have been derived from a solid rock such as forms our mountains. 

 In our first excursion we saw that chemical change promoted dis- 

 integration; in our second, running water is observed seizing upon 

 these products of decay, transporting them and building them into 

 stratified deposits in the first convenient pool. A level-topped delta 

 is first formed, which may or may not grow to fill the pool in which 

 it is born. Some of the pools have become filled, while the delta 

 as such has disappeared; it has grown into a tiny sand plain. 



Let us see if the work performed by these temporary rivulets is 

 typical of running water in general. For this purpose we shall visit 

 a spot where a river enters some considerable body of water such as a 

 lake. Let us inspect the river. Its water is sluggish, discolored by 

 organic matter derived from decaying vegetation, and for some dis- 

 tance up stream from its mouth it meanders slowly across a flat, 

 marshy area or meadow. If we also visit the spot at a time when the 

 river is swollen by heavy rains or melting snows, the presence of this 

 organic matter will be masked by the turbidity of the water; we shall 

 learn that only in the freshet seasons does the water attain sufficient 

 velocity to carry any visible load of sand and clay. The upper end 

 of the lake will be found to be shallow, muddy, and water lilies 

 will have discovered congenial surroundings. At another part of 



