BTRKEY, GEOLOGICAL RECONNOISSANGE OF PORTO RICO 43 



Minor Structures 



Although there is extensive development of sedimentary formations 

 which have been subjected to much disturbance, there is comparatively 

 little structure of a minor sort that seems to deserve such discussion in 

 this description. Two, however, that seem to have special significance 

 connected with the origin of the particular beds which have been found 

 are (a) a peculiar crumpled, enterolithic structure seen in one of the ash 

 beds near "Guayama, and (6) the wind-assorted cross-bedding structure 

 of the old dune sands of the San Juan formation at Arecibo. 



Enterolithic Structure. — The enterolithic structure noted in the ash 

 beds, on account of the thinness of the bed, — about eighteen inches, — and 

 the simplicity of the associated structure, — simple tilted beds, — leads one 

 to believe that the structure is essentially primary rather than of subse- 

 quent dynamic origin. Its appearance is perfectly consistent with the 

 explanation that it is preserved from the time of deposition and its be- 

 havior at that time as a small mud flow. It is a structure such as might 

 be formed by slumping movement of a soft layer. It should be expected 

 that there would be frequent behavior of this kind in the accumulation 

 of such extensive beds of ashy materials, which must in some cases have 

 been deposited under conditions that would make slumping movements 

 possible, but it is not to be expected that material of this kind would in 

 most cases be capable of preserving any of these primary movements. In 

 the case noted, the quality of the interior makeup of the bed seems to 

 have been more favorable to such preservation. It is the only case where 

 such an observation was made. 



Double Cross-hedding. — The cross-bedding structure belonging to the 

 San Juan formation is a prominent feature wherever these ancient dune 

 sands are preserved. A great prevalence of steeply inclined minor struc- 

 tures is crossed by fewer nearly horizontal ones. Measurements made on 

 sea-cliffs a short distance west of Arecibo, where this rock is very promi- 

 nently developed, gave dips of 30 to 33 degrees repeatedly. A series of 

 these is abruptly terminated by a more nearly horizontal bedding for a 

 comparatively short distance and the whole structure is repeated. The 

 layers with this kind of structure are prevailingly one-half to two feet 

 thick and no ripple marks were seen on any of the beds examined. The 

 cross-bedding structure in this case dips always to the west or southwest, 

 and the average strike of the principal beds is about north 30 degrees west. 

 This is consistent with a wind direction not very different from the pres- 

 ent prevailing winds. Very strong structural development of this kind is 

 also to be seen in the city of San Juan at the promontory on which the 



