105 JOURNAL OF THE 
gion have been produced. Other cascades and shoals 
are developed under the following conditions: 
Fic. 4.—Conditions favoring the develop- 
ment of cascades and rapids in river channels 
crossing areas of granitic and gneissic rock. 
gr.—Granite. 1—Fault or break in the rock, 
the right side having moved down or the left 
side moved up. 2—A schistose zone in the 
granite resulting from the shearing or move- 
ment of the rock along a line of weakness. 
gn—Gneiss, in which there are alternately 
harder and softer portions, the harder and 
more obdurate places being more heavily 
shaded (as at 4). 5—Dike of diabase or other 
material harder and more obdurate than the 
gneiss, and never producing a cascade or rapid 
in the stream channel. 
2. In, portions of the granitic area 
there are lines of structural weakness 
where, under great strain or pressure, 
the materials of which the rock is 
composed give way and are flattened 
out by a process known as shearing, 
so as to give there a rather gneissic 
or schistoese structure, as at 2 in fis. 
4. ‘The rock in this condition is often 
more rapidly attacked by the weath- 
ering and eroding forces of the atmos- 
phere, and, consequently, as the 
streams cross the surface of the 
FIG. 4. 
~ couutry where such conditions exist, they carve out their 
channels more rapidly, thus producing shoals or rapids, 
and, in extreme cases, cascades or falls. 
3. Conditions somewhat similar to the above and fav- 
orable to the formation of shoals and rapids in streams 
are sometimes found along the line of contact between 
areas of granites and gneisses, as at 3 in fig. 4; and 
