62 : REPORT—1862. 
Rain falls vertically, and tends to sink vertically into rocks, producing decompo- 
sition in them, both by mechanical and chemical action. A superficial coating of 
greater or less thickness is always thus kept in a state of decay. 
In almost all granite districts, the rock beneath the hollows and flatter parts of 
the ground will often be found to be decomposed in situ to a mere sand, so that it 
could be dug out with a spade to a depth of several feet. Roundish lumps are 
found here and there in this sand, which were the centres of the original blocks; 
these, as well as the solid rock below, showing every gradation of firmness, from 
hard crystalline rock to a mere incoherent sand. I have observed this in granite 
districts in all parts of the world, and was much struck with it during the past 
summer in the southern part of Brittany, where the deep narrow lanes often showed 
both granite and gneiss thus rotten and soft, to a depth sometimes of fifteen or 
twenty feet. On the steeper slopes the exposed rock was much less decomposed, 
obviously because the particles had been washed down and carried off as fast as they 
became completely disintegrated. 
Hard limestones, again, exhibit the effects of the action of the rain in the numerous 
open fissures and caverns that are always found in them, the water here having 
dissolved the rock and carried it off in solution, as if it were so much salt or sugar. 
The fantastic forms and honeycombed surfaces of all limestone crags attest the 
same action. In baring the surface of a limestone quarry where the beds are in= 
clined at anty donsiderable angle, they are often found to be furrowed by rain-channels 
one or two feet in depth and several inches in width, the hollows being filled with 
the finest earth. A deep covering of mould and turf is no protection against this 
action, and perhaps even aids it by contributing an additional dose of acid to the 
rain-water. 
Even where hard siliceous rocks exhibit a weathered coat of a very slight depth, 
a mere skin perhaps of a quarter of an inch thick, as is the case in some Felstones, 
still it merely proves that the atmospheric influences cannot affect a great thickness 
at any one time, and does not render it impossible that many such weathered coats 
may have been formed outside the present surface, and successively removed 
altogether by the completion of the process. 
The joints of rocks when first formed are doubtless mere planes of separation, 
without any interstice that would allow the insertion of even the thinnest edge of 
a knife ; they would be quite insensible to the sight, and would perhaps scarcely of 
themselves be sufficient to cause the separation of the rock into distinct blocks. 
In working deep mines it is sometimes said that the rocks cease to show any 
joints at all. The joints, however, doubtless exist, although they are invisible, 
while the open joints, such as we see in all rocks near the srirfabe, have been 
opened by the “ weather” acting along these concealed planes of separation. 
The action of the atmosphere, then (7. e. the chemical action of air and water 
and the various gases mingled with them, and their mechanical action, owing both 
to their movements of gravity and their expansion and contraction from changes of 
temperature), is operative in the gradual destruction of rock, to a much greater 
vertical depth beneath the surface than is commonly recognized. Its superficial 
action is still greater, and has also, as I believe, generally failed, as yet, in receiving 
due appreciation. The rain that falls upon the surface and does not sink beneath 
it runs, of course, down the shortest and steepest slopes it can find, and is collected 
first into rills, then into brooks and rivulets, and finally passes by rivers to the 
sea. This superficial drainage of a country is often augmented and kept up by 
springs, which are caused by that part of the water that had sunk beneath the 
surface finding its way back to it. 
The natural tendency of running water is to cut its channel deeper, and that at 
a rate compounded of the rapidity of the current and the nature of the rock below. 
Let any one take the basin of drainage of any great river, and trace it up to its 
source, following all its tributaries to their sources, and he will not fail to perceive 
that all the varied features of the different channels of this system of running 
waters are the result of these two circumstances only. In the mountain glens he 
will see those that traverse granite commonly with rounded open forms; those 
that cut through hard slates, or thick horizontal sandstones, are commonly narrow 
and precipitous, with jagged cliffs and overhanging ledges, perhaps, jutting from 
