28 AXXALS XEW YORK ACADEMY OF SCIEXCES 



In addition to these intrusive masses, which are of small size or at least 

 of not verv great areal dimensions, there are in a few districts large boss- 

 like occurrences of massive coarse-grained igneous rock. The boundaries 

 of none of these have been traced out. but it is certain from the distribu- 

 tion now known that in each case the area occupied is several square 

 miles in extent. The most prominent one of this type of intrusive mass 

 is that seen in the southeast portion of the island, including the district 

 about Huamacao and Yabucoa and Las Piedras and Juncos. .Whether or 

 not this is all one mass belonsfins: to a single intrusion has not been deter- 

 mined. The variety of comj^osition seen in the different samples taken 

 at different points is consistent with the presence of more than one intru- 

 sive unit : but it is also possible and quite as likely that the variety ob- 

 served is wholly due to magma tic differentiation. The southerly portion 

 of this mass, especially that near Yabucoa, is represented by a very coarse, 

 very quartzose and almost peginatitic granite. Farther to the north, in 

 the vicinity of Las Piedras and Juncos, the rock has the appearance of a 

 syenite. Although a part of the rock does show the composition of a true 

 syenite, by far the greater nimiber of specimens collected on this expedi- 

 tion show the presence of quartz in sufficient amount to make the rock a 

 granite. It would appear, therefore, that this occurrence in the south- 

 east portion of the island is essentially a granite mass and that it is of 

 unusually large size, reaching practically from the coast at ^launabo to 

 Caguas. The distance across this mass is. therefore, not less than about 

 12 miles north and south. In all probability it is not of quite so great an 

 extent east and west, but these boundaries are unknown. 



One other large intrusive mass was observed in the west central part' of 

 the island, in the vicinity of Jayuya and Utuado. In general appearance 

 and texture this rock, in the average outcrop, does not differ much from 

 that seen at the east end of the island which is commonly referred to as 

 syenite. In this occurrence, however, such specimens as have been exam- 

 ined with the aid of the microscope, show the presence of quartz in most 

 cases in sufficient amount to make the rock of granite composition. In 

 this case, as in that referred above, there are considerable differences of 

 composition shown by the rocks which seem to be a part of the same mass. 

 Specimens found, for example, near Adjuntas have the compositional 

 characteristics of diorite. whereas a specimen taken near the margin of 

 the boss on its northerly side, near K-53 on the road toward Arecibo, is 

 a syenite. At certain other j^oints near -Utuado, the rock is a granite 

 porphyry. 



The best idea of the variety of composition and textural quality repre- 

 sented by all kinds of intrusives in the island can be gathered from an 



