BERKEY, GEOLOGICAL L'ECOXXOISi^ANCE OF PORTO RICO 53 



peated at occasional intervals in successive horizons. As such a series is 

 lifted above sea level and suljjected. to ordinary erosion and weathering, 

 the tendency is, (a) for the purer and more massive reef limestones to be 

 attacked by the solvent action of percolating water with a development of 

 underground channels, porous rock condition and actual caverns, (&) for 

 the :nore earthy layers to resist and limit such action at the levels where 

 this matter is present in sufficient abundance, with a development of 

 residuary material. As this action progresses toward maturity, many of 

 the larger ca\'es collai)se and sink holes are thus formed. AVith still fur- 

 ther development, the sink holes merge into each other in local areas 

 where solution has been most active, the earthy debris forms a soil in the 

 l)ottom corresponding in level M'ith the first important earthy layer, and 

 adjacent remnants of tlic limestone reef stand out as sharp rugged hills 

 separated by irreguJar ]iotches that represent other smaller collapsed 

 caves. The result of such action and conditions, finally, is the numerous 

 "haystack'" hills standing on flat soil-covered areas or surrounding such 

 areas as if they were just set down as bunches on this surface. This re- 

 lation is repeated at different levels throughout the belt from San Juan 

 to Agnadilla. but the most striking developments are local, apparently 

 M'here the structural relations are just right, and may be seen best be- 

 tween Tao Alto and Arecibo, especially in the vicinity of Manati and 

 Vega Alta. 



It was at first thought tluit former subsidence levels might have some- 

 thing to do with establishing the level tracts, but the observation that 

 these tracts stand at very different levels in immediately adjacent dis- 

 tricts together with recognition of the structural difference, lead us to 

 give credit to the primary structural character of the formation itself as 

 the controlling factor in the present distribution. According to this ex- 

 planation, these hills are mere remnants left from solution attack on a 

 reef limestone, the depressions between them representing collapsed cav- 

 erns, the walls of which may still be seen on the sides of some of the more 

 rugged hills, and the surrounding or intervening tracts are soil-covered 

 and level, chiefly because of the accumulation of earthy material, left be- 

 hind after removal of the overlying reef, now halted in its reduction at 

 the first important less solulile beds. 



MINERAL RESOURCES ^ 



An examination of specimens of minerals and ores in the hands of 

 local prospectors and residents interested in developing mineral resources, 



"A sood list or tabulation of the mineral occurrences of Porto Rico may be found in 

 tbe article by H. C. B. Nitze listed at the close of this paper. 



