ELISHA MITCHELL, SCIENTIFIC SOCIETY 106 
again in gneissic areas in places the rocks are harder and 
more obdurate, as indicated by heavier shading at 4 in 
fio. 4; and in the beds of streams crossing such areas 
the rocks wear away irregularly, the harder portions 
standing out as projections while the intervening softer 
materials are hollowed out. In this way we have pro- 
duced a succession of shoals, a few hundred yards or 
several miles apart ; and between these are to be found 
the quiet reaches of the streams where the current moves 
along more smoothly and quietly. 
4. Another structural feature in granitic and gneissic 
areas, and also in slaty and sandstone areas, which oc- 
casionlly results in the production of the shoals and 
rapids, is the occurrence of dikes, where cracks in the 
earth’s crust have been subsequently filled with various 
materials in a plastic and usually a molten condition and 
which materials have subsequently hardened. If the 
material of the dike is softer than that of the granitic or 
g@neissic rock on one or both sides of it, then there will 
be a drop in the course of the stream from the adjoining 
rock of the wall down on to the softer dike surface, as is 
shown at 2 in fig. 4. If on the other hand ‘the material 
constituting the dike is harder and more durable than 
the materials on each side of it, the country rock on the 
lewer side of the dike, owing to its being softer and less 
durable than that composing the dike itself, will wear away 
more rapidly than the material of the dike, and consequent- 
ly the water will drop from the dike surface on to the 
country rock below it, as indicated at 5 in fig. 4 above. 
The occurrence of the belt of eruptive rocks between the 
two belts of slate, as shown in fig. 3 “p. 103), may be con- 
sidered as analogous to this last-mentioned case. On 
the Deep river near Gulf and on New Hope creek, a 
tributary of Haw river, are to be found illustrations of 
the development of waterpower being favored by the oc- 
