From such studies as have been published it appears that the rock formations of Santo Domingo correspond to the secondary, the lower and middle tertiary and the quaternary epoch. The most ancient part of the island is the central mountain range, also a series of protuberances in the Samana peninsula, the nucleus of the Baboruco mountains and a single point in the northern coast range near Puerto Plata. The tertiary lands are those forming the entire northern part of the island from the central range to the sea, portions of the Samana peninsula between the older rocks, a large area to the southwest of the Zamba hills, smaller tracts between the Jaina and Nizao rivers, and the region between the salt lakes on the Haitian frontier and between Barahona and Neiba. The modern lands are the coast plains and the small terraces on the south of the central range and on the south of the Baboruco mountains, the Maguana, Azua and Neiba valleys, small areas on the north coast at the foot of the mountains, and the marshes and Yuna River delta at the head of Samana Bay. In the central mountain range is found a nucleus of eruptive rocks which have raised and twisted sedimentary strata, covering them and forcing them aside. This nucleus is not a regular feature of the whole length of the chain, but is an irregular mass beginning about at the middle, in the region of the Jaina River, and extending in a series of parallel lines obliquely across the backbone of the range to the border of the Republic and on into Haiti. Among these rocks and bent and broken by them are the slates, conglomerates and calcareous rocks which are found in the mountains and over the whole surface of the island. The character of the central range and the inclination of the strata of cretaceous rocks make it probable that the island emerged from the sea in the eocene period, its area being then confined to the extent of the central mountain chain, with a few small islands to the south, one or more islets to the northeast, comprising the older peaks of the Samana range, and a small archipelago to the southeast, where the hills of Seibo now are. During the miocene period these islands became surrounded with coral reefs, the vestiges of which remain in strips of calcareous rock found in the same position in which they were deposited. Towards the end of the tertiary period, after a time of quiet, there was a new rise of the land. While the hills to the south of Samana Bay and the bed of the Cibao Valley from Samana Bay to Monte Cristi rose slowly, there was an upheaval further to the north, and the Monte Cristi Range was formed. Before this period it had been a bar at sea-level, covered with a clayey sediment of chalk. At a later geological period the great plains to the north and east of Santo Domingo City were formed.