But although water plays such a very important part in the making of the common rocks around us, yet there are others which have quite a different origin,--rocks which have come up from below the surface of the earth in a heated and molten condition, such as the lavas that flow from volcanoes in active eruptions and the showers of ashes and fine volcanic dust which often attend such eruptions (see chap. viii., pp. 271-272). Some highly heated rocks, though they never rise to the surface to form lava-flows, are forced up with overwhelming pressure from below, and wedge themselves into the sedimentary rocks that overlie them, thus forming what are known as volcanic dykes, and intrusive masses or sheets of once molten rock. In this category we include such rocks as basalt, felstone, pitchstone, and other rocks of fiery origin that have flowed from volcanoes as lava, as well as those like granite, which have cooled and become solid _below_ the surface, and are Plutonic, or deep-seated, igneous rocks. Granite may be exposed to the surface of the earth when the rocks which once overlaid it have been worn away or "denuded." It is frequently seen in the central regions of mountain-chains, where a vast amount of erosion has been effected. Thus we see that heat has played its part in the making of rocks; and for this reason such rocks as we have just mentioned are called _igneous_. Fire and water are therefore very important geological agents; but we should say heat rather than fire, because the latter word might convey a false impression. No rocks can be burned except coal, which may be considered rather as a mineral deposit than as a rock. Some rocks may be heated, and undergo many and various changes in their mineral composition; but they are not capable of combustion.