76 ‘ Reafons for fuppofing that 
The confideration of this fact would induce one to 
imagine, that the bed of a river undergoes infen- 
fible changes from the friction of its own ftream, 
after it ceafes to be expofed to the more manifeft 
ravages of a torrent. Buta little attention to the 
ceconomy of nature will remove the fufpicion, by 
pointing out a fimple preventive, which fhe ufes to 
obviate the inconveniency. For when a pebble 
‘has been a while depofited in water, it is covered 
with feveral of the imperfec? plants, moft commonly 
with the conferva rivularis, and different fpecies of 
tremella. ‘Thefe fingular productions of the vege- 
table kingdom are enabled by fome peculiarities of 
their conftitution to bear the friction of a ftream 
without receiving the leaft injury: And, to what- 
ever circumftance this property is owing, it is plain 
that the fubftances in queftion, while they provide 
for their own fecurity, muft enfure the protection 
of whatever they envelop. By fpreading over 
every part in contact with the water, they preferve 
one continued furface, and prefent themfelves entire 
‘and uninterrupted to the aétion of the current ; 
and by occupying the crevices arifing from the 
inequalities of contiguous ftones, they forma kind 
of cement, in which the lighter fragments being 
entangled, are prevented from being moved by 
every trivial force. ‘Thus is permanence given to 
the courfe of every river by this fimple provifion of 
nature; and bounds are fet to a procefs, which 
without it muft have been unlimited. 
I have 
