BERKEY, GEOLOGICAL RECONNOISSANCE OF PORTO RICO 21 



River at the point where the military road crosses it. A photograph of the 

 interbeddecl relation of tuff and limestone at this point accompanies this 

 discussion. This is probably, in part at least, the rock referred to by 

 Hill in some of his discussions as "mountain limestone.'" It seems to us 

 that the several occurrences of limestone beds which have clearly diifer- 

 ent relations in the series, and the probability of being able to place them 

 in different horizons, based on this content, makes it desirable to use 

 more characteristic local designations for them. Such usage cannot be 

 confusing even if it should be proven, as it may ven^ well be, that some 

 of them are identical. In this discussion, therefore, wherever possible, 

 the chief occurrences that are not clearly identical with formations 

 already described are characterized by adding the name of the locality 

 where there is especially good development of the formation. A lime- 

 stone seen at Coqui, considerably farther east, and several occurrences 

 near Yanco, and others still farther toward the west, have many points 

 of resemblance to the Coamo limestone. In some cases even the brown 

 volcanic spots are also present, but this is not true of all places. The 

 most constant index as seen in the field is a fine meshed coralline fossil 

 form, strikingly resembling a piece of loosely woven cloth. 



The limestone beds developed at Coamo reservoir, however, rarely show 

 this type, but instead have a remarkable development of algfe of the form 

 known as Lithothamnia. 



Trujillo Alio Limestone. — There are several other limestone members 

 in the older series. One has been observed only on the north side of the 

 island in the vicinity north of Trujillo Alto and in the vicinity of Loiza. 

 This is a very dense fine bluish limestone made up wholly of fine micro- 

 scopic organic growths. In some places it has a rough fragmental struc- 

 ture, but for the most part the rock is massive and the abundant organic 

 content, largely alg£e, is its most striking characteristic. Whether it is 

 younger or older than the Coamo limestone has not been determined, but 

 that it belongs essentially to the same general series is quite certain. 

 On account of its distribution it is conveniently referred to as the Tru- 

 jillo Alto limestone reef. This member probably has a very moderate 

 thickness and no great areal distribution. It is affected by solution de- 

 veloping caves at the Trujillo Alto locality. in much the same manner as 

 is the Areci])o formation, but this rock is a much more compact type and 

 its content and structural relations are quite distinct. It was probably 

 of reef origin also, but is associated intimately with the upper shale 

 members of tlie older series rather than with the Tertiary series. 



^'SJirecF Limestone. — Another limestone may be seen at several places 

 on the Arecibo-Ponce road on the south side of the divide from K-13 to 



