22 ANNALS NEW YORK ACADEMY OF SCIENCES 



K-19. It lias no great thickness, but there are several independent beds. 

 Its most striking character is the presence of patches of dark color dis- 

 tributed in a shred-like way through the grayish mass. The rock as a 

 whole is massive, exceedingly compact, of a bluish gray color, and, except 

 for these dark-colored shreds, shows no recognizable structure whatever. 

 They contain algae, however, which are expected to determine something 

 more definite about their position in the series, but they are obscure 

 forms, and doubtless considerable work will have to be put on these beds 

 to determine their exact horizon. They are intimately associated with a 

 series of igneous fragmental beds and a considerable thickness of very red 

 shale or ash beds. Together these alternating limestone and red frag- 

 mental beds make a striking structural succession which was not seen 

 anywhere else. They lie in a position which is not far from the point 

 where the Coamo limestone belt should be expected to cross the Ponce- 

 Arecibo road, but no such structural development has been noted at any 

 other j)lace. It is, of course, possible that the fine red ash represents the 

 tuffs of the regular Coamo formation and the "shred'' limestone repre- 

 sents a phase of the Coamo not developed elsewhere, but the striking 

 physical difference encourages the making of a distinction, at least for the 

 present. 



La Muda Limestone. — A rather heavy development of limestone in the 

 vicinity of La Muda between Eio Piedras and Caguas has some super- 

 ficial resemblance to the beds just described from the Arecibo road, but 

 their relationship is not fully determined. The rock is not prominently 

 tufaceous and is not marked in the same way. It has in places a coarse 

 fragmental structure almost completely obscured by healing and it is, as 

 usual, attacked by cave development. Some of the caves have collapsed, 

 leaving a complex aggregate partly made up of igneous material filling 

 the former chambers. A conglomerate bed lies below the limestone and 

 shale at this point and both are cut off abruptly by a large intrusive 

 mass. How these are related to other typical members of the older series 

 is not known, but it will be possible to trace the beds to more definite 

 relations. This is probably one of the oldest limestone members in the 

 pre-Tertiary series. It is conveniently referred to as the La Muda 

 Limestone. 



In addition to this there are ver}^ numerous small or thin local devel- 

 opments of limestone layers distributed through the shale beds at various 

 points. These are taken to be, in most cases, simply somewhat more 

 heavily developed limy layers of the same origin as the rest of the fora- 

 miniferal and ashy shales ; but the nature of their origin shows that it is 

 reasonable to expect a development of calcareous content sufficient to 

 make them more of a limestone than a shale. 



