[Illustration: Fig. 71.--The crater-lake called Lago del Bagno, in Ischia, converted into a harbour.] [Illustration: Fig. 72.--Lake of Gustavila, in Mexico. (The terraces round the lake have been artificially formed.)] [Sidenote: DIMENSIONS OF CRATER-LAKES.] Two of the largest crater-rings in the world are found in Central Italy, and are both occupied by lakes, the circular forms of which must strike every observer. The Lago di Bracciano, which lies to the north-west of Rome, is a circular lake six and a half miles in diameter, surrounded by hills which at their highest point rise to the height of 1,486 feet above the sea, while the surface of the waters of the lake is 640 feet above the sea-level. The Lago di Bolsena is somewhat less perfectly circular in outline than the Lago di Bracciano; it has a length from north to south of ten-and-a-quarter miles and a breadth from east to west of nine miles; the surface of the waters of this lake is 962 feet above that of the waters of the Mediterranean. The lake of Bolsena, like that of Bracciano, is surrounded by hills composed of volcanic materials; the highest points of this ring of hills rise to elevations of 684, 780, and 985 feet respectively above the waters of the lake. In these great circular lakes of Bolsena and Bracciano, as well as in the smaller ones of Albano, Nemi, and the lakes of Frascati in the same district, the vast circular spaces enclosed by them, the gradual outer slope of the ring, and the inner precipices which bound the lake, all afford evidence of the explosive action to which they owe their origin. But while the vast crater-rings we have mentioned are frequently found to be occupied by lakes, there are many other similar crater-rings which remain dry, either from the materials of which they are composed being of more pervious character, or from rivers having cut a channel through the walls of the crater, in this way draining off its waters. Thus in the Campi Phlegræi, while we have the craters of Agnano and Avernus forming complete circular lakes, Astroni has only a few insignificant lakelets on its floor, and the Pianura, the Piano di Quarto, which have each a diameter of three or four miles, with many others, remain perfectly dry. In the vicinity of the great crater-lakes of Central Italy we find the crater-ring of the Vallariccia, which has evidently once been a lake but is now drained, its floor being covered with villages and vineyards. [Sidenote: CRATER-RINGS SURROUNDING CONES.]