38 



places, essentially the same as the layers below, down even to the 

 very rock, so that we might call it all soil, though, since a special 

 name, regolith (meaning stone blanket), has been proposed for all 

 the soft, soil-like rock-cover, we may speak of it as regolith and 

 reserve the word soil for the surface layers only. 



In some places there is no soil on the bare rocks ; elsewhere the 

 soil-cover is a foot or two in depth ; but there are places where the 

 regolith is several hundred feet deep. In such places, even the 

 wells do not reach the rocks ; nor do the streams cut down to it ; 

 but even there, if one should dig deep enough, he would reach the 

 solid rock beneath. 



How has this hard rock been changed to loose soil ? One of the 

 wa^'S, of which there are several, may be easily studied whenever a 

 rock has been exposed to the air. Let us go to a stone wall or 

 among the pebbles in a field, for instance, and, chipping off the sur- 

 face, notice how different the inside is from the outside. The outer 

 crust is rusted and possibly quite soft, while the interior is hard and 

 fresher. Many excellent examples of this may be seen in any stony 

 field or stone wall. 



As hard iron rusts and crumbles to powder when exposed to the 

 weather, so will the minerals and the rocks decay and fall to bits ; 

 but rocks require a very much greater time for this than does iron. 

 It happens that the soil of I^Tew York has not been produced by the 

 decay of rock ; and, therefore, although most soils of the world have 

 been formed in this way, we will not delay longer in studying it 

 now, nor in considering the exact way in which rocks are enabled 

 to crumble. 



Another way in which rocks may be powdered may also be seen 

 in most parts of New York. The rains wash soil from the hillsides 

 and the streams become muddy. In them there are also many peb- 

 bles, representing the larger fragments that have fallen into the 

 stream after having been broken from the ledges. The current 

 carries these all along down the stream, and, as they go, one piece 

 striking against another, or being dragged over the rocks in the 

 stream bed, the pebbles are ground down and smoothed (Fig. 16), 

 which means, of course, that more mud is supplied to the stream, as 



430 



