52 



ANNALS NEW YORK ACADEMY OF SCIENCES 



San Juan. They constitute a feature so unusual tliat even the un- 

 trained casual visitor is impressed with them. 



This feature has been referred to before in connection with the de- 

 scription of the "Younger series" of rocks, especially the Arecibo reef 

 limestone formation. In spite of the unusual appearance presented by 

 this distribution of "haystack'^ hills and intervening flats, their origin is 

 judged to be comparatively simple. The active agents and processes 

 have been the same as those at work on all other parts of the island, but 

 the results differ l^ecause of the fundamental difference of material and 



Fig. 20. — Care styictiire in the haystack (pepino) hills 



Near view of the limestone hills forming the margins of the small cultivated flats in 

 the typical haystack hills district. This view shows the cavernous nature of the lime- 

 stone forming these hills, a structure that is regarded as the most significant feature 

 and prohahly the largest factor in the development of these peculiar relief forms. 



structure. Xowhere are these features developed except Avhere the later 

 reef limestones are the underlying bed-rock formation. 



The essential steps in the development of these forms are the follow- 

 ing : 



The reef limestones are not uniform in composition or structure. 

 They have more or less intermixture of earthy matters which are distrib- 

 uted irregularly, but chiefly at certain horizons, as more earthy or shaly 

 beds of no verv a'reat lateral or vertical extent. Such conditions are re- 



