_Mountains supply many of our wants._ Besides water and soil, how many useful things come from the hills! Their slopes, watered by the clouds, frequently support an abundant growth of pine forest; and thus we get wood for the shipwright and joiner. Again, mountains are composed of harder rocks than we find in the plains, and that is one reason why they stand out high above the rest of the world. Their substance has been hardened to withstand for a longer time the destruction to which all rocks are subjected. They have been greatly compressed and generally more or less hardened by subterranean heat. We bake clay and make it into hard bricks; so Nature has baked and otherwise hardened the once soft strata of which mountains are chiefly composed, converting them into slates, schist, gneiss, and other kinds of rock called "metamorphic" by geologists, because they have been altered or metamorphosed from their original condition (see chapter viii., page 277). Again, granite, basalt, and other rocks known as "igneous," which once existed in a molten condition, have forced their way up from subterranean regions into the rocks forming mountain-chains; and a good deal of the hardening just alluded to is due to the presence of these fiery intruders, which have baked and hardened the rocks around them to a considerable extent, altering at the same time their mineral composition. The same causes which led to the injection of granite, basalt, and other igneous rocks in mountain-ranges brought other consequences in their train. Whatever the causes, they were closely connected with volcanic eruptions, so that highly heated water and steam found their way through cracks and other fissures in the rocks; and in the course of time the chemical actions thus set up led to the deposition of valuable metallic ores within these fissures. In this way mineral veins were formed; and volcanic action seems to be largely responsible for the production of minerals. Thus we find around Vesuvius, and in fact in all volcanic regions, large and varied supplies of minerals. Now, the geologist discovers that many mountain-chains--such, for example, as the Grampians, Alps, and Carpathians--have in past geological periods been the seats of volcanic action on a grand scale; and so we need not be surprised to learn that mountainous countries yield large supplies of valuable gems and metallic ores (see chapter viii., page 277). Even in the days of Solomon, the active and business-like Phoenicians were carrying on trade with Great Britain; and the tin came from Cornwall. Besides tin, gold, silver, lead, copper, zinc, and other metals come from our hills. Now, however, we get our copper mostly from the Andes, and our gold from Australia or South Africa, because it can be got more cheaply from these countries, to which many of our Cornish miners have emigrated.