"You have estimated very well," was the reply. "The northern island is called Chincha, and gives the name to the group, and it is about the length and width you mention. The other two are smaller, but are of the same formation as Chincha, a bright red granite composed of red feldspar, white quartz, and a little mica. The group is evidently of volcanic origin, and perhaps it may one day disappear beneath the waves as other volcanic islands have done. "Guano can only accumulate where there is no rain," continued their mentor, "and there is another source of wealth here that comes from the rainless district." "What is that?" "It is the nitrate of soda," answered the Doctor, "which comes from several desert regions in the southern part of Peru, chiefly in the province of Tarapaca, which has been annexed to Chili since the war, and is Peruvian territory no longer. It has many uses in industrial arts, and is largely employed as a fertilizer; the deposits have been worked since 1830, and the chief points of export are Iquique and Pisagua. In twenty years from 1830 the exports were 240,000 tons, and in 1875 no less than 326,000 tons were exported. In 1877 there were 253 ships that cleared from Iquique alone with cargoes of nitrates. Several of the railways constructed by the Peruvian government, or on private account, were built partly or wholly for the transportation of this article." The steamer stopped very briefly at Pisco, and there was not time to go on shore. From Pisco to Mollendo they were almost constantly in sight of the coast, and sometimes hugging it closely; the mountains of the western cordillera of the Andes filled the eastern horizon, and occasionally the snowy peaks of the great central chain were visible. The principal chain of mountains in South America is called the Andes, and sometimes the _Nevadas_ (white), to distinguish it from the cordillera (cor-de-_yer_-ra), by which the lateral and lower chains, generally parallel to the Andes, are designated. _Sierra_ (from the Spanish word for saw) is a spur, or irregular line, of mountains stretching from the Andes to the cordillera, or pushing out from the latter into the flat _Parama_, or desert. [Illustration: ON THE EDGE OF THE DESERT.] Mollendo is the ocean terminus of the railway to Arequipa and Lake Titicaca, the present destination of the boy travellers and their elder companion. The town is on the edge of the desert, and the harbor is an open roadstead, like most of the ports of the western coast. An old captain sarcastically remarked, "the harbor of Mollendo is entered as soon as the ship turns Cape Horn." The town is supplied with water by an iron pipe eighty-five miles long, which starts from near Arequipa, and is capable of discharging 430,000 gallons of water every twenty-four hours. Enormous tanks have been constructed, to maintain a supply for several days, in case of accident to the aqueduct, and these tanks are the principal sights of the place.