The Apennines of Tuscany are of very unequal height, and they are traversed by numerous low passes, which could easily be converted into carriage roads. Speaking generally, they consist of a series of elongated and parallel mountain masses, separated from each other by valleys, through which flow the head-streams of the Serchio and the Arno. The first important mountain mass of the main chain near the frontiers of Liguria, which is commanded by the Orsajo and Succiso, is thus separated by the valley of the Magra from the parallel range of Lumigiana. The chain of Garfognana, to the north of the plains of Lucca, has for its pendant the Alps of Apuana. Monte Cimone, farther east, and the other summits of the _Alpe Apennina_ to the north of Pistoja and Prato, are attended by the parallel ridges of the Monti Catini and Monte Albano, on whose slope is the famous grotto of Monsummano, with a thermal spring. A fourth mountain mass, that which the direct road from Florence to Bologna crosses in the Pass of Futa, has likewise its lateral chains, viz. the Monte Mugello, to the south of the Sieve; the Prato Magno, encircled by the Upper Arno; and the Alps of Catenaja, between the Arno and the Tiber.[83] The Apennines of Tuscany in many places attain a height of 5,000 feet, and are quite Alpine in their aspect, the upper slopes remaining covered with snow for more than half the year. They owe much of their grandeur to the precipitous slopes and fantastic profiles of the calcareous rocks which enter so largely into their composition. The forests of chestnuts, firs, and beeches which formerly clothed the whole of the range have not yet been entirely destroyed. The beautiful woods which cover the slopes of Prato Magno have impressed the mind of many a poet; and, since Milton sang the delights of Vallombrosa, the “shaded vale” has become a proverbial name for everything sweet and touching in the poetry of nature. Farther to the west the monastery of the Campo di Maldulo (Camaldoli) occupies one of the most beauteous spots in all Italy, the woods and meadows of which have been celebrated by Ariosto. From the summit above the convent both the “Tuscan and the Slavonian Sea” can be seen, as that poet tells us.