42 . THE ROYAL SOCIETY OF CANADA 
increases in velocity to a mile and an eighth an hour, it becomes capable 
of carrying particles of double the volume, while if the velocity is de- 
creased from a mile and an eighth to one mile an hourits transporting 
power is cut in two. 
In this connection it must be borne in mind that the effective 
weights of different substances are not the same in water asin air. For 
instance, quartz which has a specific gravity of 2-6 has a specific weight 
in water of only 1.6, while gold which has a specific gravity of 19 has 
a specific weight in water of 18. Gold is therefore 7-3 times as heavy 
as quartz in air, while it is 11-25 times as heavy as quartz if weighed 
in water. It is therefore the specific weight in water of different sub- 
stances which must be considered in connection with their transportation 
by water, rather than their relative weights in air. If the specific 
gravity is constant, the diameter of the pebbles which a stream can 
carry will vary as the square of the velocity, and if the velocity of 
the stream remains constant, the size of the pebbles will vary accord- 
ing to the specific weight of the substance composing them weighed 
in water. Thus, if one pebble is of quartz and another is of gold, which 
is 11-25 times as heavy as quartz weighed in water, the volume of a 
pebble of quartz which can be carried by the current will be 11-25? or 
126 times as great as that of a pebble of gold and, assuming both 
pebbles to be cubes the diameter of the pebble of quartz will be 
approximately five times the diameter of the pebble of gold. 
With pebbles of quartz and gold of equal size it will take a current 
of V5 or 2-24 times the velocity to move the gold which it will take to 
move the quartz. 
Again, if pebbles of quartz and gold of equal size are dropped into 
water the latter will sink to the bottom with more than three times the 
velocity of the former. 
Now the quantity of material, or load, which can be collected by 
running water from the ground when it is covered with vegetation is 
relatively small, but when vegetation is absent and weathered rock 
is exposed to the direct influence of the rain and running water the load 
will often be very large, and consequently streams which carry away 
the wash from bare but weathered surfaces are often loaded to their 
utmost capacity. 
In addition to the work done by water in carrying away weathered 
and softened rock, the streams themselves cut down their channels 
into the hard unweathered rock. 
Where the water in the streams is clear and carries but little sedi- 
ment this cutting action is very slow, or almost nil, but where it carries 
any considerable amount of sediment this sediment is pushed along 
