The truth of this will appear when we reflect that the moisture which rises from the sea and falls on the land as rain, is free from any admixture of impurities; but by the time it again reaches the sea, after a circuit of perhaps many miles down valley and plain, it has grown turbid and discoloured, carrying with it a quantity of mud, sand, and drift-wood. The sediment thus transported soon sinks to the bottom, where it eventually hardens into rock, and in course of time is raised above the waves as part of a new land. Such I conceive to have been the origin of the sand, gravel, and imbedded plants of our boulder. It may be well, however, in going into the details of the subject, to take a wider view of this interesting branch of geology, and look for a little at the forms and modes of the decomposition of rocks, and the varied manner in which new sedimentary accumulations are formed. All over the world, wherever a land surface spreads out beneath the sky, there goes on a process of degradation and decay. Hills are insensibly crumbling into the valleys, valleys are silently eroded, and crags that ever since the birth of man have been the landmarks of the race, are yet slowly but surely melting away. It matters not where the hill or plain may lie, the highest mountains of the tropics and the frozen soil of the poles, yield each in its measure and degree to the influence of the general law. It might seem that so universal a process should be the result of some equally prevalent and simple cause. But when we set ourselves to examine the matter, we find it far otherwise. The waste of the solid lands, in place of arising from some single general action, is found to result from a multiform chain of causes, often local in their operation and variable in their effects. Such an investigation affords a good illustration of the general mode and fashion in which Nature delights to work. It shows us that what may seem a very simple process may be in reality a very complicated one; that in truth there exist in the world around us few if any simple, single processes, which stand out by themselves unconnected with any other; that, on the contrary, all become intimately linked together, the effects of one often forming part of the chain of causes in another, and producing by their combined action that complex yet strikingly harmonious order that pervades all the operations of Nature. To an extent of which Cicero never dreamed, there runs through all the world "such an admirable succession of things that each seems entwined with the other, and all are thus intimately linked and bound together."[51] Man separates out these various processes, classifies and arranges them, because from the imperfection of his mental powers he cannot otherwise understand their effects; all would seem but chaos and confusion. But the formal precision and the sharp lines of demarcation exist only in his mind. They have no place in the outer world.