BERKEY, GEOLOGICAL RECOXXOLSSAXCE OF PORTO RICO 19 



cially along the divides toward the west. At the two points first men- 

 tioned, in particular, they are light yellowish or reddish in color, rather 

 porous in strnctnre, strongly bedded and have a prominent blocky frac- 

 ture habit. The exact character of these rocks is a question under study 

 at the present time, but enough has been done to show that, in their 

 present condition, they have been oxidized to the 3^ellow or red color, and 

 have been leached so that they have a porous structure and light weight 

 due to the removal of at least a part of some constituent that is more 

 readily soluble than the rest of the rock, ]\Iicroscopic comparison with 

 beds of the same structural relation, but of very dark color and very 

 dense habit, leads to the opinion that the two types are not essentially 

 different in origin, but that the lighter colored and lighter weight shales, 

 such as are found at Fajardo and at Mayaguez, are simply the weathered 

 equivalents of darker ones. It appears from this comparison that the 

 shales are normally highly calcareous and that the lime content is sup- 

 plied by the presence of a very large amount of organic matter in the 

 form of foraminifera. In some cases this organic matter makes up fully 

 one half of the rock and in all cases weathering produces a very porous 

 effect that should be expected to be identical with the red and yellow 

 shales occurring typically at Fajardo and Mayaguez. The siliceous con- 

 tent of all of the shales examined proves to be exceedingly fine and wholly 

 lacking in granular or quartzose material such as characterizes most sedi- 

 mentary shales. It is the judgment of the writer that this material in 

 the shales of Porto Rico, instead of being the ordinary disintegration 

 products derived from the weathering of ordinary land masses, is in 

 reality largely ashy material of volcanic origin. With this conception of 

 them, it would appear that even the limy shales are therefore close rela- 

 tives of the ash beds, and it is entirely possible that they do not represent 

 any great difference in history, but rather somewhat different surromid- 

 ings during accumulation. 



. Limestones. — Besides the shales, there are massive limestone beds of 

 several different types. 



In most cases the occurrences are separated by structural complexities 

 that make it uncertain about field correlation, but undoubtedly later field 

 study will connect some of these and additional paleontologic study will 

 arrange their succession. The most prominent occurrences seen are de- 

 scribed below. 



Coamo Tnff -Limestone. — The limestone with the closest genetic re- 

 semblance to the types already described is represented in a broad belt 

 passing from south to northAvest across the upper end of Coamo Eeservoir 

 near Coamo Springs, and which can be traced in prominent develojunent 



