These facts are admirably illustrated in the case of the volcanic district of the Lipari Islands, to which we have had such frequent occasion to refer. The great central volcano of this district, which now in a ruined condition constitutes a number of small islets (see the map, fig. 81, p. 192), is composed of andesitic lavas. The other great volcanoes thrown up along the three radiating lines of fissure are composed of andesitic and trachytic rocks. But all the more recent ejections of the volcanoes of the district have consisted either of rhyolites, as in Lipari and Vulcano, or of basalts, as in Stromboli and Vulcanello. Von Richthofen and the geologists who most strongly maintain the generalisations which he has made concerning the order of appearance of volcanic products, go much farther than we have ventured to do, and insist that in all volcanic districts a constant and unvarying succession of different kinds of lavas can be made out. It appears to us, however, that the exceptions to the law, as thus precisely stated, are so numerous as to entirely destroy its value. The generalisation that in most volcanic districts the first ejected lavas belong to the intermediate group of the andesites and trachytes, and that subsequently the acid rhyolites and the basic basalts made their appearance, is one that appears to admit of no doubt, and is found to hold good in nearly all the volcanic regions of the globe which have been attentively studied. The Tertiary volcanic rocks of our own country, those of North Germany, Hungary, the Euganean Hills, the Lipari Islands, and many other districts in the Old World, together with the widespread volcanic rocks of the Rocky Mountains in the New World, all seem to conform to this general rule. [Sidenote: THEORY OF VOLCANIC MAGMAS.] In connection with this subject, it may be well to refer to the ideas on the composition of volcanic rocks which were enunciated by Bunsen, and the theoretic views based on them by Durocher. Bunsen justly pointed out that all volcanic rocks might be regarded as mixtures in varying proportions of two typical kinds of materials, which he named the 'normal trachytic' and the 'normal pyroxenic' elements respectively. The first of these corresponds very closely in composition with the acid volcanic rocks or rhyolites, and the second with the basic volcanic rocks or basalts. Durocher pointed out that if quantities of these different materials existed in admixture, the higher specific gravity of the basic element would cause it gradually to sink to the bottom, while the acid element would rise to the top. Carrying out this idea still further, he propounded the theory that beneath the earth's solid crust there exist two magmas, the upper consisting of light acid materials, the lower of heavy basic ones; and he supposed that by the varying intensity of the volcanic forces we may have sometimes one or the other magma erupted and sometimes varying mixtures of the two.