Coasting steamers with about twelve feet draught cross the bar of the Rio Grande and lie at Aparri. The river is navigable in the dry season as far as Alcalá for light draught steamers. Alligators abound in these rivers. In this valley, which extends through the provinces of Cagayan and Isabela up into Nueva Vizcaya, there is to be found a great extension of rich alluvial soil on which can be raised, besides other tropical crops, most excellent tobacco, the cultivation of which was for many years obligatory upon the inhabitants, who were forbidden to grow rice. Little has been done in the way of making a trigonometrical survey of the Highlands of Luzon, but some military reconnaissance sketches have been made from time to time by staff or engineer officers employed in building forts, and from these several maps have been compiled. One of the most complete of these is by that intrepid explorer and painstaking geographer, D'Almonte. Another map has been published by Colonel Olleros. It must be admitted that these maps do not agree with each other, but that is not unusual in maps of the Philippines, and results from a custom of the Spanish engineers of doing too much in the office and not enough in the field. Colonel Olleros has, however, on his map shown the lesser known mountain ranges very vaguely, and has left more than a thousand square miles of territory quite blank. This tract lies between the central range and the Cagayan River, and is inhabited by the Apayaos, Calingas, Aripas, and Nabayuganes. Olleros also leaves some large blanks on the east coast, and he is quite right to do so, for this coast has hardly been visited since Salcedo sailed past it at the time of the Conquest, and nothing is known about that part of the island which remains to this day in possession of the savage Dumagas, a Negrito tribe. That coast is almost entirely destitute of shelter, and is exposed to the full force of the Pacific surf. It is made more dangerous by tidal waves which are formed either by distant cyclones or by submarine upheavals and occur without warning. The largest and richest valley in Luzon is that which extends without a break from the shores of the Bay of Lingayen to the Bay of Manila, having an area of some 3000 square miles, and comprising the best part of the Provinces of Pangasinan, Tarlac, Nueva Écija, Pampanga, Bulacan and Manila. The town of Tarlac is situated about half-way between the two bays, and approximately marks the watershed. About half-way between Tarlac and the northern shores of Manila Bay there rises from the plain an isolated mountain of volcanic origin, Mount Arayat. The crater has been split through and the mountain thus shows two peaks. It is covered with forest to the very summit. Arayat was thrown up within historic times, and the Indians have a tradition that it was completed in one night, which is a most unlikely story.