BERKEY, GEOLOGICAL RECONNOISSANCE OF PORTO RICO 39 



crumpled and overturned in a most complicated manner. Minor fold 

 structures of this kind are especially prominent in the higher ranges, for 

 example, along the Jayuya road along the divide opposite station K-24r 

 of the Ponce-Arecibo road. At such a place may be found as compli- 

 cated structural features of this kind as is usually present in any folded 

 mountain region. The high angle at which such beds stand at many 

 other points leads to the belief that similar complexities characterize a 

 great many of the districts occupying the higher mountain portion of the 

 island as well as some of the marginal areas, but the great amount of 

 erosion and the very limited exposures at most points tend to obscure 

 some of these details. The complex way in which the igneous intrusive 

 masses cut all of these formations also tends to obscure and modify and 

 further complicate the simpler fold structures so that it is not always 

 possible to properly credit the disturbed attitude. On account also of the 

 fact that the total quantity of injected or intruded materials, including 

 dikes, sills and bosses, is exceedingly large and must have caused exten- 

 sive disturbances by reason of the displacement produced by the occupa- 

 tion by these intrusive masses, it is likely that much of the observed ab- 

 normal attitude of the bedded rocks may be due to this cause rather than 

 to regional folding of a simpler sort. It is fair to say, however, that a 

 sufficient amount of data is not yet available to draw general conclusions 

 as to the meaning of the fold structures in Porto Rico. The striking 

 thing is that all of the older formations are disturbed and that their 

 position and attitude, even along the margins of the island, indicate that 

 the region affected by these movements was more extensive than the 

 present land area. 



Faulting 



There are many evidences of faulting on a small scale, in some of which 

 the displacements can be measured. But in most cases the direct evi- 

 dence lies chiefly in the existence of crush zones, slickensided walls and 

 abrupt changes of rock tjrpe; there is no opportunity, on account of the 

 general rock complexity, to secure quantitative data. Judging from the 

 difficulty in tracing certain formations between districts where they have 

 been identified, it is probable that there are occasional faults of large dis- 

 placement. J^umerous crush zones were seen on the Comerio road espe- 

 cially, but in this case also no system was discernible from the few meas- 

 urements available. 



The most prominent fault, in its effect upon present features, is the 

 one now marking the inner margin of the younger series of chalky lime- 

 stones and shales constituting' the coastal belt along the south side of the 



