But in the remainder of Luzon, from the isthmus of Tayabas eastward and southward, the coast line is most irregular, and the width much reduced. A chain of mountains commencing at and forming the two above-mentioned islands and running in a south-easterly direction forms the peninsula of Tayabas. Another range, starting near the Bay of Sogod, runs a little south of east as far as Mount Labo (1552 metres), turns south-east, and runs along the southern shore of the fertile valley of the River Vicol, and with many a break and twist and turn reaches Mount Bulusan, whose slopes run down to the waters of the Strait of San Bernardino. The convolutions of this range form on the south the secure harbour of Sorsogon, and on the north the bays of Albay, Tabaco, Lagonoy and Sogod, besides a multitude of smaller ports and bays, for the coast line is wonderfully broken up by spurs of the main ranges running out into the sea. Luzon generally is very mountainous, and more especially so that part lying to the north of 16° 5', where the great ranges of mountains run in crooked lines but with general north and south direction. The range running parallel to the Pacific coast is called, in its most southern part, the Caraballos de Baler, and the rest of it, up to Punta Escarpada, is known as the Cordillera del Este, or the Sierra Madre. The central range, starting from Mount Caraballo in the latitude before mentioned, is called the Cordillera Central for about a degree of latitude, and from there is known as the Cordillera del Norte, terminating at Punta Lacatacay, in longitude 121° east of Greenwich. The mountains on the western coast are not so lofty, nor do they form a connected range. They are known as the Sierras de Ilocos. Some of these ranges are thirty or forty miles long. There are cuts in places where rivers find an outlet to the sea, such as the Rio Grande de Laoag, the Rio Abra, and some lesser streams. All these ranges have spurs or buttresses. Those of the Central Cordillera extend as far, and join with, the coast range on the west, covering the whole country and leaving no large plain anywhere, for the valley of the Abra though long is very narrow. There is a little flat land about Vigan. But the eastern spurs of the central range, in the part of Luzon under consideration, do not interlace with the spurs of Sierra Madre, but leave a magnificent valley more than two degrees of latitude in length and varying breadth. This is the only great valley of northern Luzon, and through it runs the Rio Grande de Cagayan and its tributaries, the Magat and the Rio Chico, with numerous minor streams.