BERKEY, GEOLOGICAL RECOXNOISSAXCE OF PORTO RICO 45 



dent that some peculiar condition has been in control in the development 

 of this compound structure. There is no reason to assume any difference 

 of source or origin for the steeply inclined structures, or cross-bedding 

 structure, from that assumed for all of the other occurrences of the San 

 Juan formation. It is apparently a perfectly normal product of the as- 

 sorting action and deposition of material under the work of the wind. 

 But mider normal conditions it Tvould not happen that a second structure 

 in a horizontal position should be repeatedly developed crossing the well 

 marked cross-beds so that the whole complex combination should be de- 

 veloped on such a scale as is seen in this case. From the nature of the de- 

 posit and the conditions in which it may well be assumed to have been 

 formed — that is^, at or near sea level in the vicinity of or bordering upon 

 standing bodies of water — one would be inclined to favor the explanation 

 that the accumulating cross-bedded sands fell into or rolled into a body 

 of water which had a tendency to attack the newly deposited material and 

 to bind the grains together. The difficulty with such an explanation is 

 in the fact that the horizontal structure is repeated at small intervals 

 practicalh' throughout the deposit, and apparently without disturbing 

 the primary depositional structure at all. It would seem quite unlikely 

 that loose matters of this kind, falling into or rolling into a l)ody of water 

 of an open surface sort, should maintain or preserve the primary struc- 

 ture so well. Perhaps it is more logical, in view of all the features, to 

 connect the development of this structure which seems, from its slight 

 influence or modification of the cross-bedding, to be wholly secondary 

 with the subsidence of the coast which is sho^vn to have been one of the 

 late events in^the geological history. As subsidence progressed, perhaps 

 somewhat irregularly, it would liappen that the ground water level would 

 rise correspondingly high in beds that were passing below sea level. At 

 the surface of this ground water level the tendency would be to accomplish 

 a binding of the granular materials together. Both above and below 

 the ground water level there would probably not be so strong a tendency 

 to develop this binding. With the next step in the progress of subsidence, 

 another streak or indurated zone would be established and these have 

 been repeated throughout the whole formation during the wliole period 

 of subsidence. An action of this kind would not tend to disturb the pri- 

 mary structure at all; it would on the contrary tend to preserve it or 

 make it less destructible because of the improved induration. But it 

 could, in addition, develop a succession of secondary structures through- 

 out the whole mass which, if the binding is fairly substantial, might rival 

 the primary structure in prominence when exposed to subsequent de- 

 structive attack. It is possible that such a succession of liorizontal struc- 



