GEOLOGY AND MINERAL RESOURCES. The island has three geological elements: (1) A central sys tem of deeply ribbed and corrugated mountain: with V-shaped gorges and ridges; (2) lower hills along the north and south coasts; (3) coastal plains with alluvial soil extending from the foot of the central mountains across the line of foot hills to the sea. The central mountains are formed of volcanic eleeta—tuffs and eonglom erates—with occasional dikes and interbedded Midst! limestone, which, however, is of rare oc currence. These mountain rocks, which are of Cretaceous and possibly early Eocene age, are entirely decayed at the surface, breaking down into a red clay resembling that of the southern Appalachians. The foothills are all of later Ter tiary and Pleistocene age, and are composed en tirely of rocks of sea origin. consisting of the peculiar type of tropical white limestones of a chalky, marls•, and shelly nature. The island abounds in clay suitable for ordinary brick and earthenware. Good building sand is found, but little of it is suflic•iently pure for glass-making. The white limestones of the coastal plain supply excellent lime. The building stones are volcanic boulders and limestone. Douses made of boulders have a picturesque and nibbled appearance. Those constructed of limestone are always stuccoed. and the most elaborate buildings are made of lime stone, including all of the public buildings and fortresses. Beautiful marble of great hardness is quarried near .Juana Diaz, but as yet is used only for structural purposes, such as bridge piers.
Sandstone is comparatively rare, but fine flagstone is abundant all over the island. Gypsum is used extensively for stucco, plaster, and fertilizer. A large area of the southern coast abounds in phos phates, but the industry has developed only on Mona island. off the west end of Porto Rico, where it is estimated that there are nearly 500.000 tons of guano and phosphates in the caves already explored. Only about 50,000 tons of these fer tilizers have as yet been extracted. Year Ponce and elsewhere numerous eaves arc filled with rich deposits of guano, which are now being worked. Lignite is found. but the fuel resources have not yet been investigated. Sufficient salt for the needs of the island may easily he obtained from many lagoons near the sea by natural evaporation of the brine. The principal deposits are the salines of Coama. Guanica, and Sierra de Pifiones, de Cabo Rojo. invalids resort to a number of mineral springs. chiefly at Ponce and Coamo.
No mining of metals is yet carried on. Those most frequently found are gold, carbonate and sulphide of copper, and magnetic oxide of iron. Since the discovery of the island, gold has been washed in small quantities from many rivers, especially in the north and east. Experience has shown that the placer deposits are not rich or extensive. At one place north of Juncos there is a large deposit of magnetic iron ore.