436 
ICAL IM OM se) B: 
[ Feb. 24, 1870 
the walls are so absolutely vertical, that it is impossible to 
find a pathway between their base and the water. But 
where, owing to rapids, some portage was necessary, the 
explorers usually succeeded in carrying their stores, and 
sometimes even their boats, along the base of the cliffs. 
The water of the Colorado River is red and muddy. 
It receives some tributary streams of clear water, but 
others are very turbid, particularly one which the expedi- 
tion appropriately marked as the Dirty Devil. More- 
over, after every heavy shower of rain, “cascades of red 
mud pour over the walls from the red sandstone above, 
with a fall of hundreds of feet.” We await with interest 
the detailed report which Colonel Powell will furnish of 
these features of the river. 
Dr. Newberry, who described this territory in the 
report of the former Exploring Expedition above referred 
to, declared his opinion that, notwithstanding the stupen- 
dous scale on which these canons or ravines had been 
formed, they were all nevertheless true river-gorges, ex- 
cavated by the erosive action of running water. Some 
geologists, as Dr. Foster of Chicago, in his recent work 
on the Mississippi Valley, have opposed this opinion, and 
have suggested that “the form and outline of these 
chasms were first determined by plutonic agency.” But 
Dr. Newberry’s explanation has been very generally ac- 
cepted. He showed that there is nowhere any trace of 
fracture or disturbance, and that when the Canon is dry 
its rocky bottom shows no mark of dislocation. In- 
deed, when we consider the intricate ramifications of 
these cafions, so precisely similar to the ordinary outlines 
of a drainage system over a low flat ground, it seems 
impossible to conceive of any agency capable of pro- 
ducing such ravines save the streams which flow in them. 
But if canons are merely the results of ordinary river 
erosion, why do they not occur everywhere? To sucha 
question we may reply that river-ravines do occur every- 
where, but it is only where the special circumstances 
which favour the formation of such ravines are most fully 
developed that they grow into the depth and length of 
canons. What then are these special circumstances ? 
If we watch what takes place along the course of the 
rivers of this country, we can mark two kinds of erosion 
distinctly at work. First there is the river, grinding 
down the sides and bottom of its channel by sweeping 
along sand and shingle; and, secondly, there is the 
action of rain, springs, and frosts perpetually loosening 
the sides of the water-course, and sending the débris into 
the river which sweeps it away. If the river were not 
interfered with by these other subaérial agents, it would 
in time dig out for itself a gorge with more or less pre- 
cipitous sides. But in proportion as these agents come 
into play, the ravine-like character passes into that of a 
valley with sloping sides. Where river erosion pre- 
dominates we have ravines, where it is modified by rains 
and springs, but especially by frosts, we have valleys. 
Many of our rivers run both through gorges and along 
valleys, the changes in the nature of their banks being 
determined by corresponding changes in the nature and 
grouping of the rocks of which these banks consist, and 
the greater or less facility with which the rocks have been 
worn away by the one form of denudation or the other. 
The conditions needful for the formation of cafions, there- 
fore, appear at present to be chiefly these :—1st. The 
erosive power of the streams must be greatly in excess of 
that of the other forms of atmospheric denudation. The 
rainfall must be small, or, at least, so equally distributed 
over the year as to reduce pluvial action to a minimum. 
Frosts must be equally rare and unimportant. The main 
streams drawing their supplies of water from a distance, 
either from melted snow or abundant rainfall in the 
upper parts of their basins, must be maintained in suffi- 
cient volume to keep their channels full, either for the 
whole, or a good part of the year. 2nd. There must be 
a considerable uniformity in the character of the rock 
which the stream has first to cut through. It is not 
necessary that the rock should be soft, but it should pre- 
serve for a long distance, and present to the erosive 
action of the river, the same kind of geological texture 
and structure. Hence, horizontal or gently undulating 
strata, as of sandstone, or limestone, offer the greatest 
facilities for the erosion of canons, as we know they do in 
our own country for the formation of ordinary river- 
ravines. When once the river has excavated its channel 
so deep that it cannot quit it, the nature of the rock may 
vary indefinitely without materially altering the aspect of 
the canon. Hence on the Colorado, while the upper and 
chief part of the cafon has been cut through flat sand- 
stone, limestone, and other strata, the lower portion has 
been excavated in marble and even in granite. 3rd. The 
country must be sufficiently elevated above the sea, either 
originally or by subsequent upheaval, to permit of a con- 
siderable declivity in its river-channels. The slope must 
be sufficient, not merely to let the water run off, but to give 
rise to currents strong enough to sweep along sand and 
gravel, and to excavate pot-holes. It is by the ceaseless 
grinding of such detrital material along the bottom of the 
river that the ravine is slowly deepened. Geologists, 
although they have constantly recognised this action, 
have not, perhaps, been always fully aware of its rapidity 
and extent, partly, no doubt, from the want of reliable 
data as to the nature and amount of the detritus pushed 
by rivers along the bottom of their beds. Messrs. Hum- 
phreys and Abbot computed that the Mississippi annually 
pushes into the Gulf of Mexico 750,000,000 cubic feet of 
gravel and sand, ‘‘which would cover a square mile about 
twenty-seven feet deep.” The writer of the present paper 
was surprised a few years ago to find that the Rhine, after 
escaping from all its ravines and entering the low country 
about Bonn, retained force enough to drive along shingle 
upon its bed. By laying the ear to the bottom of a boat 
floating down mid-channel, it was easy to hear the grating 
of the stones as they rolled over each other. Hence we 
see that a river, which may be perfectly navigable by 
steamers, may yet have rapidity enough to scour its bed 
with coarse shingle. The scour will, of course, be greater 
in proportion to the narrowing of the breadth of the 
stream and the increase of the slope. 
It is mainly this eroding action which, so far as we 
know at present, has carved out the cafions of the 
Colorado. These wonderful ravines, meandering as ordi- 
nary rivers do, have sunk inch by inch into the country, 
retaining their original curves and windings, though con- 
tinually increasing in depth. Unassisted, or aided but 
feebly, by the other subaérial agents, which, in such a 
country as ours, tend to break down the walls of ravines ; 
and undisturbed by the inequalities of surface so charac- 
teristic of regions that have been under the influence of 
glacier-ice,* the rivers, probably once much fuller than 
now, have been allowed to dig out their gorges through the 
table-lands of the Colorado, and to convert a tract of 
country, originally, perhaps, green and well-watered, into 
a dreary desert, intersected by a network of profound im- 
passable ravines. ARCH. GEIKIE 
SCIENTIFIC SERIAL 
Revue des Cours Scientifiques, February 19.—This num- 
ber contains a list of subscribers to the Sars Fund ; also a lecture 
delivered at the Sorbonne by M. A. Cazin, on ‘‘Motive Power,” 
in which are described the laws obtaining in regard to those 
natural forces which are already made available as sources of 
motive-power and the application of some other forces which 
may probably be turned to account in the same way as science 
progresses ; for instance, the application by M. Mouchot and 
Ericsson of solar heat for working a steam engine is especially 
mentioned as worthy of consideration ; and the application of 
the force of tides suggested by M. Tommasi. 
* The absence of any trace of glacial action on the Pacific slope is noted 
by Whitney (Proc. Acad. Nat. Sciences, California, iii. 272), and by Foster 
(“ Mississippi Valley,” p. 338). 
