BERKEY, GEOLOGICAL RECONNOISSAXCE OF PORTO RICO 13 



material and which have been the object of some exploratory work for 

 coal. It is evident, from observations made, that the shale beds of the 

 Arecibo formation vary greatly and in some places are entirely wanting. 

 At the Arecibo Eiver, for example, where the Ijeds of the formation can 

 be seen well exposed in the river bluffs, there is no shale development at 

 all. The limestones lie abruptly on the eroded and somewhat weathered 

 surface of the older formations which at this point are represented by 

 coarse and obscurely bedded or even massive volcanic tuffs. At the best 

 place seen, which was on the east side of the river about opposite K-66 on 

 the Arecibo road, there were a few feet of transition material between the 

 limestone beds proper and the unmodified tuff. It had not the structural 

 appearance and makeup of the shale as seen at other points, however, 

 and this part of the formation is regarded as entirely absent on the Are- 

 cibo Eiver. Shaly beds, however, are seen again on the south side of the 

 island and their best development is in the vicinity of Juana Diaz, where 

 some of the beds are distinctly sandy and rather fossiliferous and carry 

 petrified wood. Lignite is also reported from this vicinity, but no ma- 

 terial, of that kind was found by the writer. It is more than likely, that 

 the shale beds on opposite sides of the island do not correspond in horizon 

 at all, but that structurally they are both l)asal beds. 



The most striking development of the shale beds and overlying marls 

 and softer layers of thin-bedded character, instead of the massive reef 

 structure, is on the Jacaguas Eiver south of Juana Diaz. The dips also 

 of the formation in this particular locality are much greater than those 

 observed at any other point. For considerable distances an average dip 

 of 30 to 36 degrees was estimated and the total thickness represented, 

 based upon the width of the belt, must be at least 3,000 feet. At no 

 other point on the south side, hoAvever, was there an opportunity to see 

 whether the beds of this character are constant or of large lateral extent. 

 As one goes eastward, a comparatively short distance, they are almost en- 

 tirely lacking. On the Descalabrado Eiver, which is only ten kilometers 

 to the east, the underlvino^ older series of tuffs and intruded shales and 

 limestones were followed to a distance of two miles south of the military 

 road, whereas at Juana Diaz the basal shales of the Areci1)0 formation 

 begin a half mile above the military road. It appears, therefore, that the 

 formational margin is swinging rapidly southward, and it is judged, 

 from other observations made, that there is almost nothing of it repre- 

 sented at a distance of twenty kilometers to the east, or. in other words, 

 that the formation docs not extend farther east than the vicinity of Sa- 

 linas. At one other point on the south margin of the island, there is an 

 unusuallv good opportunity to follow the successions ol' formations, and 



