At the end of the last century a great controversy took place on the question of the origin of rocks, and the learned men of the day were divided into two parties. One of these parties, following the teaching of Werner, professor of mining at Freyburg, who inspired great enthusiasm among his disciples, declared that all rocks were formed by the agency of water. This was a very sweeping and of course rash conclusion. But whenever they examined rocks, they found so many clear evidences of the action of water that a powerful impression of the importance of this agency was naturally made on their minds. They found rocks uniformly arranged in great layers which extended for long distances, and containing the remains of animals which must undoubtedly have lived in the seas or estuaries. These layers were further divided into smaller layers, such as clearly were formed by the slow settling down of sand and mud. Others again contained gravels and rounded pebbles, testifying in no uncertain way to the action of water. Even the little grains of sand are obviously water-worn. This teaching was quite sound so long as they confined their attention to clays, sandstones, and limestones; but when they came to basalt and granite, a blind adherence to the views of their master caused them to shut their eyes to the clear evidences of the action of heat, presented by such rocks. The crystalline structure of such rocks; their irregular arrangement, often so different from the uniform disposition of the stratified rocks (although it must be admitted that ancient lava-flows often lie very evenly between aqueous rocks), and the way in which they burst through overlying rocks, thus proving their former molten condition; the signs of alteration exhibited in the aqueous rocks into which they intruded themselves (changes which are obviously due to the action of heat),—these and other evidences were entirely overlooked, and Werner declared that basalt had been found as a sediment under water.
This school of geologists, believing so strongly in the all-powerful influence of Father Neptune, received the not inappropriate title of "Neptunists."
On the other hand, the party who happened to be in districts where granite, basalt, and such igneous rocks abounded were equally impressed with the importance of the powerful agency of heat. To them nearly every rock they met with seemed to show some signs of its action. And since Pluto was the classical deity of the lower regions, and the earth shows evidences in places of greater heat below the surface, this party received the title of "Plutonists;" and so the battle raged hotly for some time between the Neptunists, with their claims for cold water, and the fiery Plutonists of the rival school of Edinburgh, with their subterranean heat. Fire and water are never likely to agree; and they did not do so in this case. But now that the battle is over, and both sides are found to have been partly right and partly wrong,—though the Neptunists have the advantage,—we can afford to smile at the fierceness of the contest, and wonder how it was that each side thought they were so entirely in the right.
Let us now consider the aqueous rocks, and see if we can gain a clear idea of the ways in which they were formed; and first, we will take those of a purely sedimentary origin,—the sandstones, pebble-beds, gravels, and clays. These, as the reader has already probably guessed, have all been transported by means of streams and rivers, and settled down quietly in seas at the mouths of rivers or in inland lakes. There is no trace of the action of heat in the forming of these rocks, though they often show signs of having suffered more or less change from contact with highly heated igneous rocks of later date which forcibly intruded themselves from below; and if the change thus effected were considerable, we should call the rocks so altered metamorphic. But we are now dealing with their original state and how they were made; and of that there is no possible doubt whatever. So for the time being we may call ourselves Neptunists.
Streams and rivers are the great transporting agents whereby the never-failing supply of débris from the waste of the land is unceasingly brought down from the mountains and hills, through the broad valleys and along the great plains, until finally it is flung into the sea. The sea is the workshop where all the sedimentary rocks are slowly manufactured from the raw material brought to it by the rivers. But for the present we must confine our attention to the question of transport. Referring back to our illustration of the cathedral, we may say that streams and rivers play the part of cart and horses. They bring the materials down from the quarry to the scene of action,—the workshop where they are wanted. The quarries, in this case, may be said to be almost everywhere. For wherever rocks and soil are exposed to the action of wind and weather, there is certain to be more or less decay and crumbling away. But it is among the hills and in the higher parts of the mountains that the forces of destruction are most active. How this is brought about will be discussed in the seventh chapter, on the carving of the hills. The frequent slopes covered with loose stones are sufficient evidence of the continual destruction that takes place in these regions.
The transporting powers of rivers are truly prodigious. Looking at a stream or river after heavy rain, we see its waters heavily laden with mud and sand; but it is difficult to realise from a casual glance the vast amount of material that is thus brought down to lower levels. If we could trace the sediment to its source, we must seek it among the rocks of mountains far away. Step by step we may trace it up along the higher courses of the river, then along mountain streams rushing over their rocky beds, tumbling in cascades over broken rocks, or leaping in waterfalls over higher projections of rock, until we come to the deep furrows on the sides of mountains along which loose fragments of rock come tumbling down with the cascades of water that run along these steep channels after heavy rain, leaving at the base of the mountain great fan-shaped heaps of stones.
"Oft both slope and hill are torn
Where wintry torrents down have borne,
And heaped upon the cumbered land
Its wreck of gravel, rocks, and sand."