TRANSPOETATION AND EROSION. 19 



between the particles. Under favorable circumstances the interspaces by 

 degrees become so large that minute sand or clay particles are carried 

 along by the water, and thus mechanical attrition helps to enlarge still 

 more the passages between the grains of earth. In numerous wells in the 

 glacial till the water has been reported as being found in "gravel." I have 

 examined several such wells and found that subterranean waters had 

 percolated through the till until they had carried off the finer particles, 

 leaving the larger stones somewhat rounded b}" the flow. I infer that -when 

 the till was first formed the water percolated through all parts of the mass 

 at a nearly uniform rate. By degrees the seeping became more rapid along 

 certain lines or layers, where there was the lai-gest water supjjly or the 

 most matter readily removable. These layers soon became more porous 

 than the rest of the till and formed a system of subterranean streams or 

 "veins." In my early studies of the till I was often puzzled at these 

 apparently water-washed beds of gravel in what would otherwise be 

 amorphous till. This phenomenon occurs in the granitic and clay-slate 

 regions as well as elsewhere. In such regions the surface waters do not 

 sink down into the till in large streams, like the sinks of a limestone region, 

 and the till is in most cases, perhaps in all, compact enough to thoroughly 

 filter the water before it has penetrated vaanj feet. The presence of muddy 

 water in a deep well that is protected from surface wash around its mouth 

 indicates subterranean erosion rather than access of muddy surface waters. 

 Such cases have happened to my knowledge. However, this erosion is 

 rarely so rapid as to muddy the water perceptibly. Ob^"iously the longer 

 the process continues the more porous the subterranean channels become, 

 and the escape of the waters will be more rapid with correspondingly 

 rapid erosion. 



Vr'^hen water is flowing through a porous stratum, especially of sand, 

 with such velocity as to overcome the mutual adhesion of the grains and to 

 carry them along with it, we have what is known as quicksand. In like 

 manner, gravel will flow like a liquid if water flows rapidly through it. 

 This is the cause of the very great amount of erosion effected by what are 

 known as "boiling springs." I have elsewhere recorded instances of large 

 areas — square miles— of porous gravel eroded and removed by boihng 

 springs assisted by surface waters. When a stream impinges against a 

 gravel bank, the stones b}^ their mutual adhesion jjrotect one another from 



