The stunted vegetation of these sterile plains was sufficient to nourish large animals of the pachydermata tribe, now extinct, even at a period when the present shell-fish of the Patagonian seas existed. The Pampas of Buenos Ayres are entirely alluvial, the deposit of the Rio de la Plata. Granite prevails to the extent of 2000 miles along the coast of Brazil, and with syenite forms the base of the table-land. The superstructure of the latter consists of metamorphic and old igneous rocks, sandstone, clay-slate, limestone, in which are large caverns with bones of extinct animals, and alluvial soil. Gold is found in the alluvial soil on the banks of the rivers, and diamonds, so abundant in that country, in a ferruginous conglomerate of a very recent period. The fertile soil of the Silvas has travelled from afar: washed down from the Andes, it has been gradually deposited, and manured by the decay of a thousand forests. Granite again appears, in more than its usual ruggedness, in the table-land and mountains of the Parima system. The sandstone of the Andes is found there also; and on the plains of Esmeralda it caps the granite of the solitary prism-shaped Duida, the culminating mountain of the Parima system. Limestone appears in the Brigantine or Cocollar, the most southern of the three ranges of the coast-chain of Venezuela; the other two are of granite, metamorphic rocks, and crystalline schists, torn by earthquakes and worn by the sea, which has deeply indented that coast. The chain of islands in the Spanish main is merely the wreck of a more northern ridge, broken up into detached masses by these irresistible powers. CHAPTER X. Central America—West Indian Islands—Geological Notice. TAKING the natural divisions of the continent alone into consideration, Central America may be regarded as lying between the Isthmus of Panamá and Darien and the Isthmus of Tehuantepec, and consequently in a tropical climate. This narrow tortuous strip of land, which unites the continents of North and South America, stretches from S.E. to N.W. about 1200 miles, varying in breadth from 20 to 300 or 400 miles. As a regular chain, the Andes descend suddenly at the Isthmus of Panamá, but as a mass of high land they continue through Central America and Mexico, in an irregular mixture of table-lands and mountains. The mass of high land which forms the central ridge of the country, and the watershed between the two oceans, is very steep on its western side, and runs near the coast of the Pacific, where Central America is narrow; but to the north, where it becomes wider, the high land recedes to a greater distance from the shore than the Andes do in any other part between Cape Horn and Mexico. This country consists of three distinct groups, divided by valleys which run from sea to sea, namely, Costarica, the group of Honduras and Nicaragua, and the group of Guatemala.[59]