THE SCIENCE OF OBSERVATION. 463 



If we examine the pool critically, it will be noticed that its shore 

 line is cut by a little channel along which the overflow makes its 

 escape. Further investigation will show that at another point along 

 the shore, especially if we are fortunate enough to visit the locality 

 very soon after a rain, there is a small rivulet entering the pool; and 

 also that the entering stream is discolored with mud and carries more 

 or less sand, while the escaping stream is nearly clear, and is free 

 from all traces of coarse, sandy material. It is therefore evident 

 that the sediment brought in by the stream has been left behind in 

 the pool, and of course will be found deposited at its bottom, and 

 it will appear that the only explanation of the inability of the water 

 further to transport its burden is to be found in the fact that water 

 loses nearly all its motion, and therefore its transporting power, on 

 entering a stagnant pool. These are elementary truths, but an 

 amplification of such simple phenomena is often fully capable of 

 accounting for the most stupendous results. 



Having made these observations, let us look at the form assumed 

 by the sediment when it is forced to fall to the bottom. At the point 

 where the stream enters the pool there is seen an accumulation of 

 material having a nearly level upper surface, presenting a scalloped 

 or lobe-shaped outer margin, upon which the stream may be seen 

 flowing and entering the water at one of the lobes. Other channels, 

 though unoccupied by water, also lead to similar lobes. If We watch 

 closely, we may be able to witness the growth of this body of sand, 

 called a delta, as the falling sediment rapidly increases the size of the 

 lobe; and also to perceive that as soon as the lobe is built out con- 

 siderably in advance of the main body of sand, it will be easier for 

 the stream to enter the water on one side of the scallop, thus aban- 

 doning its old mouth. In this manner the stream moves from one 

 place to another, successively building the little scallops and continu- 

 ally carving new channels for itself. Fig. 3 is a photograph of 

 such a delta, some three feet across, taken after the water had been 

 drained away, and reveals its form in a characteristic manner. As 

 we watch its growth, it will become evident that only the coarsest 

 material transported by the stream goes to make up the delta, and 

 that the clay and finest sand are deposited farther awy, where the 

 water is more quiet, or else pass out in the stream draining the pool. 

 Let us look about a little. JSTot far from our miniature lake there are 

 several others. In some the size of the delta is much larger in pro- 

 portion to the area of the pool than is the case with the one first 

 studied. We find in some cases that the stream has progressively 

 built its delta completely across the old water surface. Taking a thin 

 piece of board or a large knife, we can easily cut vertically through 

 this sand deposit, thus exposing what is called a geological section. 



