BERKEY, GEOLOGICAL REGONNOISSANCE OF PORTO RICO 35 



ments made of earth along roads and trails behave in the same way and 

 one is continually surprised at the steepness of such slopes and their ap- 

 parent stability. Slopes of 40° are not rare on hillsides that are culti- 

 vated, and one occasionally crosses divides that are mere knife edges with 

 slopes of this kind on both sides. Such stability of the soil is a great 

 factor in preserving the agricultural productivity of Porto Eico and in 

 the distribution of its agricultural industries over so much of the in- 

 terior area. 



There seem to be three factors of large consequence in this stability of 

 the soil mantle. One is the clinging character of some of the vegetation 

 which tends to bind the soil together ; another is the small range of tem- 

 perature variation which reduces disintegration or disruption tendencies 

 to a minimum ; and still another is the low content of inert or refractory 

 materials, such as quartz, in the rocks whose destruction has furnished 

 the soils; all of which factors favor the making of especially tenacious 

 soil. Most of the soils are for the same reason exceedingly difficult to 

 cultivate. As a direct consequence of this soil behavior and climatic 

 control, there is comparatively little dust formed in Porto Eico. This is 

 especially noticeable on the roads, where one is almost never in the least 

 troubled by it. 



In two districts, one on the north fringe of El Yunque along the 

 Sabana Eiver and the other on the north side of the divide near Adjuntas, 

 deposits were seen which suggest glacial action. Very large boulders are 

 stranded in jiositions where it is difficult to account for them by ordinary 

 erosion means, but too little study was given and too little evidence is at 

 hand to warrant a more definite statement. 



STRUCTURAL FEATURES 



Most of the structural features represented in the geology are men- 

 tioned in connection with descriptions in other sections of this report. 

 This is especially true of such structures as may be regarded as essen- 

 tially primary, including the interbedded and intrusive forms of various 

 kinds. There is no doubt but that the most prominent structural com- 

 bination in Porto Eican geology is represented by the succession of inter- 

 bedded sedimentary beds and tuffs, cut liy or interleaved with intrusivei 

 in the form of sills, dikes or irregular stringers. The combination occurs 

 in great varietv as to succession, relative amounts or proportions, quality 

 of material, present condition and relations to other members, but in 

 spite of these variables the structural feature is essentially the same and 

 has the same meaning everywhere. 



