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CHAPTER III. LAVAS. Lavas. Under this head are a variety of rocks largely employed for building purposes on the Con tinent, from the most ancient down to modern times. Such is the case in Central France, the districts of the Lower Rhine, the Moselle, and in Italy— countries where these rocks have been erupted from volcanoes either extinct, or in activity. These lavas (from which we omit basalt and dolerite, already described) are generally highly sili- cated, and come under the general terms of trachyte and phonolite, or clinkstone ; the latter, however, is rarely the product of modern volcanic activity. 1 Trachyte. (From rpayoy, rough.) Spec. grav. 2.4— 2.8 ; contains silica 50.67 per cent. This name was first proposed by Hauy in the year 1822 for certain rocks of Auvergne, 2 and was soon generally adopted for volcanic rocks, having a rough felspathic base formed of sanidine, oligoclase, with accessories such as augite and mica, silica or hornblende. When there are crystals or grains of free quartz, or sani- 1 B. von Cotta, Eng. edit. p. 182. 2 Zirkel, Petrog. ii. 141.