RECORDS OF MEETINGS 279 



Mr. Semmes stated that the San Juan district lies on the northern 

 side of the island of Porto Eico. It extends from the city of San Juan 

 on the east to a point about two miles east of Manati and southward as 

 far as Barranquitas — an area approximately 500 square miles in extent. 

 The San Juan district is a typical N-S section through the northern 

 half of the island, the geological history of which may be regarded as 

 essentially the same as that of the whole island. The physiographic 

 history of the San Juan district is that of a complex mountainous old- 

 land, which has been peneplained, partially submerged, overlapped by a 

 coastal plain, uplifted, maturely dissected, again slightly submerged and 

 partially uplifted; erosion continued in the interior from the time of 

 the first uplift. The formation of the district may be divided into a 

 younger and an older series. The younger formation consists of a coastal 

 deposit of indurated lime dune sand of presumably Pleistocene age 

 which overlies unconformably a limestone series. This limestone series, 

 termed the Arecibo formation, rests almost horizontal (dip, 6° N.) 

 and is now maturely dissected, giving rise to the typical pepino or hay- 

 stack topography of the island. The Arecibo formation is regarded as 

 probably wholly Upper Oligocene in age. In the northwestern portion 

 of the island, but not appearing in the San Juan district, is a lignitic 

 shale member (the San Sebastian shale) which underlies the Arecibo 

 formation. The age of this shale member is uncertain, but in all proba- 

 bility it is of Lower Oligocene age. The coastal plain has been carved 

 out of the younger series. The older series consists of marine and vol- 

 canic sediments with numerous intrusives. It is highly tilted and locally 

 folded, resting unconformably below the younger series. The sedi- 

 mentary types represented are limestones, shales, sandstones, conglom- 

 erates, tuffs and ashes. All of these sediments are derived almost wholly 

 from volcanic sources. Even the limestones show brecciated structure 

 and are, with few exceptions, mere accumulations of limestone frag- 

 ments derived from older beds shattered by volcanic outbursts. The 

 foraminiferal content of certain shale and limestone beds in the upper 

 part of the older series indicates that the beds are Upper Cretaceous 

 and Eocene in age. The lower part of the series is no doubt Comanchic. 

 The peneplaination of the older series, prior to the deposition of the 

 coastal plain, culminated therefore in late Eocene time. The older 

 series is everywhere intruded by igneous rocks of many varieties. The 

 predominant type is an andesitic porphyry. The tuffs and other sedi- 

 ments directly derived from volcanic sources are also of this general com- 

 position. The igneous rocks occur as extrusive sheets, sills, dikes and 



