Notes on the forest conditions of Porto Rico . margin of San JuanHarbor. GEOLOGIC FORMATION AND SOILS. In the Southern United States and the Antilles, where altitude isnot a controlling factor, the chemical and physical composition of thesoils are two of the chief factors producing vegetal differences. Inas-much as the soils of Porto Eico, with the exception of that of theplay a plains, are all residual (the surface decay of the underlyingrock), it is impossible to make a clear presentation of the forest condi-tions without a few remarks upon the nature of the rocks. It is notthe intention to
Notes on the forest conditions of Porto Rico . margin of San JuanHarbor. GEOLOGIC FORMATION AND SOILS. In the Southern United States and the Antilles, where altitude isnot a controlling factor, the chemical and physical composition of thesoils are two of the chief factors producing vegetal differences. Inas-much as the soils of Porto Eico, with the exception of that of theplay a plains, are all residual (the surface decay of the underlyingrock), it is impossible to make a clear presentation of the forest condi-tions without a few remarks upon the nature of the rocks. It is notthe intention to go into geologic detail, but inasmuch as all culturaland natural aspects are intimately associated with geologic structure,a few words upon this subject are absolutely essential. PI. Ill showsthe undergrowth and substructure on the river Brujo. GKEOLOGY. The mountains are composed largely of black or other dark-coloredbasic igneous rocks, occurring as tufts, conglomerates, and silts of tin No. 25, DV. of Forestry, U. S. Dept. of Ag Plate SOILS. 15 horublende-andesite, cut by dikes of diorite. While these rocks are ofigneous origin, there are nowhere any signs of recent or late eruptivevolcanism, such as craters, unburied lava flows, cinder cones, etc., allorigiual 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 sedimentsiv 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 ofthese, found on the crest of the island near Oayey and Aibonito, is ablack bituminous shaly limestone interbedded with the volcanic con-^merate. This calcareous horizon, fully 1,000 feet thick, apparentlypholds the crest of the sierra and weathers into soils noted as theDest tobacco lands on the island. The other is a light-gray c
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