To the eastward through this gap, and in a broad, deeply eroded valley, runs the tumultuous Reventazon River, and to the westward the Rio Grande de Pirris. On the south of this depression the Chirripo Grande mountain mass sends off east and west two immense flanking ranges. A part of the western range, lying between San Marcos and Santa Maria, for a length of about six miles, is known as the Dota ridge, to which former explorers gave great importance. This lofty, transverse and precipitous mountain system almost forbids communication between the northern and southern halves of the Republic, and, as Colonel Church says, must at all times have had a marked influence on the movement of races in this part of Central America. Both the northern and Talamanca sections present mountains in masses instead of sierrated like many Andean chains of North America. Those of the Talamanca section are Rovalo (7050 feet), Pico Blanco (9650 feet). Chirripo Grande (11,850 feet) and Buena Vista (10,820 feet). There are no signs of recent volcanic activity in the Talamanca range. The Talamanca mountains have narrow crests and are very precipitous on the Atlantic side, with evidences of extensive denudations and erosions caused by the ceaseless rain-laden trade-winds. Professor William M. Gabb, in his geological sketch of Talamanca, observes that the geological structure of the entire region is very simple. The greatest expanse is occupied by recent sedimentary rocks raised and nearly entirely metamorphosed by the action of volcanic masses. At several points along the Atlantic coast, there are found masses of rocks of still later date. Professor Gabb maintains that the nucleus of the great Cordillera of the interior is formed by granites and syenites, which, like the sediment that covers them, are broken through here and there by dikes of volcanic origin identical with the eruptive material found on a greater scale in the northern part of Costa Rica. The syenites are intrusive and have their culminating point and greatest development in the Pico Blanco or Kamuk, a mountain of great altitude, unusual ruggedness and scarred with deep and precipitous cañons. All these dikes are of more modern formation and are porphyritic. Professor Gabb also notes a thick deposit of conglomerates and sandstones, schists and limestones, the schists being the most abundant; although the conglomerates, found all over the region, indicate the previous existence of an older sedimentary formation.