SOILS. 15 



horublende-andesite, cut by dikes of diorite. While these rocks are of 

 igneous origin, there are nowhere any signs of recent or late eruptive 

 volcanism, such as craters, unburied lava flows, cinder cones, etc., all 

 origiual volcanic forms of topography having been destroyed by ero- 

 sion, to which is due the present features of configuration. Besides, 

 iniWi of this volcanic material has been worked over into sediments 

 iv rehistoric ages and now occurs in well-defined strata. 



.lcluded in this mass of volcanic rocks are two limestone formations, 

 ± L .erbedded with them and relatively inconspicuous in area. One of 

 these, found on the crest of the island near Oayey and Aibonito, is a 

 black bituminous shaly limestone interbedded with the volcanic con- 

 ^merate. This calcareous horizon, fully 1,000 feet thick, apparently 

 pholds the crest of the sierra and weathers into soils noted as the 

 Dest tobacco lands on the island. The other is a light-gray crystalline 

 limestone with cretaceous fossils (Eudistes). It outcrops on an east 

 and west line from near Oabo Eojo to 15 kilometers north of Ponce on 

 the Adjuntas road, and has no special agricultural value, but the nat- 

 ural vegetation is always noticeably different where these rocks occur. 



The surface of the upper part of the pepino hills is made up of a 

 rather hard lime marl full of coral heads, with occasional indurated 

 strata of firm white porous limestones. These rocks (the pepino for- 

 mation) are of Miocene age, as determined by Mr. T. Wayland Vaughan 

 from the corals collected by the writer, similar to certain rocks of An- 

 tigua 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 West Indies in late geologic time. 



Below this limestone, which is at least 100 feet thick, are fossiliferous 

 greensand marls of undetermined age (Eocene or Oligocene), which in 

 turn rest upon a great thickness of thinly stratified reddish lignitic 

 clays and sands of Eocene age (the Richmond formation), which out- 

 crops near San Sebastian, Guatemala, and Mocha on the western end 

 of the island and near Carolina on the northeast coast. 



The south coast hills are composed entirely of 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 Oligocene. 



The i>laya plains are composed entirely of alluvium, derived mostly 

 from the mountain formations, but also mixed with the debris of the 

 adjacent white limestone hills. 



SOILS. 



The chief and radical differences of flora in Porto Eico occur between 

 the red clay mountain soils and the calcareous foothill soils, the latter 



