The evidences of such pressure are many. In some cases fossils have been thereby pulled out of shape and appear considerably distorted; in others, even hard quartz pebbles have been considerably elongated (see chap. ix., pp. 315-316). Then again, we have the little crumplings of all sizes so frequently seen in mica-schists. And lastly, the peculiar property that slates possess of splitting up into thin sheets is found to be due to the same cause; namely, lateral pressure. Slates were originally formed of soft dark mud, and on being subsequently squeezed, by earth-movements, have assumed a structure known as "cleavage," whereby their tiny mud-particles were elongated, and all assumed the same direction, thus giving to the rock this peculiar property of splitting. It can be proved that the pressure came in a direction opposite to that of the planes of cleavage; and it is found that the direction of the cleavage corresponds in a general way with the direction, or trend, of a mountain-chain which is composed partly of slates, as in North Wales. And this discovery helps and harmonises with what we have already said about the cause of the folds in mountain-chains, for the same force, acting sideways, produced the cleavage and the folding, etc. It has been already stated that in a large number of cases a mountain-range has a central axis, or band, of granite or other crystalline rock. This led some people to suppose that the granite had been driven up from below, and in so doing had thrust up the overlying rocks seen on either flank of the chain; in other words, they believed granite to have been the upheaving agent. And even now we often find unscientific writers speaking of the volcanic forces of upheaval. Having very little idea of the true structure of mountains, they believed them to consist of a kind of core, or axis, of this igneous rock, with sedimentary rocks sloping away from it on each side. This was a very simple theory of mountain-chains, but unfortunately it will not bear examination. It takes no notice of the folding which is so characteristic of mountain strata, and is quite out of agreement with the facts of the case; so it must be buried among the archives of the past. Mountain-chains are now known to have a much more complicated structure than this,--thanks to the labours of many subsequent observers.