ingredients, they produce no slight amount of physical change on the surrounding districts, and must be regarded as important geological agents. But perhaps the most common and widely-diffused form of decomposition, is that produced on the surface of the earth by the action of rain-water, in slowly dissolving out the soluble parts of rocks, and washing away the loose, incoherent grains that remain behind. It is hard to say whether this process is more chemical or mechanical. The solution of the mineral matter belongs to the former class of changes, while the removal of debris must be ranked among the latter. The results of these combined forces form one of the most important branches of investigation which can occupy the attention of the physical geologist, and in contemplating them, we are at a loss whether most to admire their magnitude, or the immense lapse of time which they must have occupied. It may be worth while to look at the progress of this kind of disintegration, that we may see how wide-spread and constant is the waste that goes on over the world, and how materially the effects of running water are by this means increased. A volume might be written about the decay of rocks, and a most interesting one it would be, but its authorship would devolve rather on the chemist than the geologist.[52] We can do no more here than merely glance at one or two illustrative examples. [Footnote 52: A German chemist, Bischoff by name, has written two learned volumes in which this subject is discussed (translated into English, and published by the Cavendish Society), valuable for their facts, but not always very safe in their deductions.] Among the mineral substances that most readily yield to the action of the weather, are the silicates and the carbonates. The rocks containing the former belong in large measure to what we call the igneous class, such as the granites and traps; while those containing the latter form the bulk of our useful stones, such as limestone and sandstone. The removal of alkaline silicates is due to their conversion into carbonates, which are readily soluble in water. Rain falling on a rock in which they are largely present, dissolves a small portion, and carries it into the soil or into streams, and thence to the ocean. Every shower in this way withdraws a minute amount of mineral matter, and tends to leave the harder insoluble grains of the rock standing out on the surface in the form of a loose pulverulent crust, easily washed away. The debris thus formed, where allowed to accumulate, makes an excellent kind of soil known to the Scottish farmers as "rotten rock."