The pebbles which form the conglomerates are composed of metamorphic clay, having a character distinct from all the other rocks found in the country. The cement is also clay or sand. The absence of crystalline rocks in the conglomerates is irrefutable proof that, when these were deposited, the syenites and granites had not yet appeared from the interior of the earth. The limestone and sandstone represent a less developed geographical horizon of the sedimentary group, the latter appearing occasionally in layers, interstratified with conglomerates or more recent schists. In no place in Talamanca have fossils been found in these sandstones, although the same rocks are very fossiliferous near Zapote on the River Reventazon. In regard to fossils, Professor Gabb saw at Las Lomas Station, about seven hundred feet above the sea, in the Bonilla Cliffs cutting, shark’s teeth, compact masses of sea shells, fish, etc., and at an elevation of 2500 feet large deposits of compact shell limestone. The schists have a fine, leaf-like texture, and are easily decomposed and reduced to a black mud, if they have not been metamorphosed. In this rock fossils have been found which belong to a Miocene age. Along the Talamanca coast calcareous deposits are found in horizontal layers, and are probably elevated coral reefs, a rock which Professor Gabb calls “antillite,” and which is developed in the entire Caribbean region. It belongs to the post-Pliocene formation, the last of the Tertiary series. In the interior valleys a thick deposit of pebbles and clays of recent origin is observed. The limit between the syenites of the high mountains and the metamorphosed Miocene formation is found in proximity to the Depuk River. In the slopes of the hills the schists are usually decomposed and covered with red clay, a sub-soil above which is found a small cap of fertile vegetable mold. In the valley of Tsuku the schists are profoundly altered and transformed in a magnesic or semi-talcous rock. The schists are more silicified in coming near to the limits of the syenites. Higher up, the granitic rocks extend in the direction of the Pico Blanco without interruption. The Pico Blanco itself is of granite. Three hundred feet below the summit porphyry is observed, while the summit itself shows a greenish-brown trachyte with black spots.