National Geographic : 1899 Mar
PORTO RICO VOLCANIC TUFF FORMATION OF THE MOUNTAINS The upper part of the Pepino hills is made up at their surface of a rather hard lime marl full of coral heads, with occasional indurated strata of firm white porous limestones. These rocks (the Pepino formation) are of Miocene age, as determined by Mr T. \Vayland Vaughan from the corals collected by me, similar to certain rocks of Antigua hitherto not known in the geologic sequence of the Great Antilles. Their tilted position, standing at 1,200 feet where they meet the older volcanic mass, testifies to the great geologic movements which have taken place in the \Vest Indies in late geologic time. Below this limestone, which is at least 100 feet thick, are fossil iferous greensand marls of undetermined age (Eocene or Oligo cene), which in turn rest upon a great thickness of thinly stratified reddish lignitic clays and sands of Eocene age (the Richmond formation) which outcrops near San Sebastian, Guate mala, and Mocha on the western end of the island, and near Carolina on the northeast coast. The south coast hills are composed entirely of chalky or other loose-textured glaring white limestone of a very porous character, often chalky, which was deposited around the margin of the mountainous island mass when it was submerged about 600 feet lower than it stands at present. These are largely of Pleistocene age, although some of the lower strata may be as old as the Oligo cene. Their surface is often covered by the peculiar efflorescent calcareous precipitate known in Mexico as tepetate, which forms a shallow subsoil or pan.