STRUCTURE OF THE TURKS ISLANDS 261 



aeolian sandstone, a discussion that will lead up to the treatment of 

 the causes of the consolidation of the rock in the mass. The harden- 

 ing process of the crust extends to a depth varying usually from 

 three to twelve inches. When, as often happens on the lesser cays, 

 large level surfaces of the hard crust are fully exposed to the sun's 

 heat, the rock becomes fissured in all directions, and the ground 

 looks as if it had been irregularly paved. With the continuation 

 of the fissuring process and the constant widening of the cracks 

 through the weathering agencies, the ground is ultimately strewn 

 with slabs of all sizes that generally vary in thickness between three 

 and twelve inches and lie in confusion around. All over these 

 islands we find in the interior and even at the coast, both on level 

 ground and on the tops of the more barren ridges, this slabby broken 

 ground in all stages of disruption. The hard crust separates readily 

 in large slabs from the underlying loose sandstone. On Penniston 

 Cay, a low flat island only thirty feet in height, the hard surface 

 crust is in places so much honeycombed and broken up that walking 

 is difficult. In other places, again, we have large slabby undulating 

 surfaces only in the early stage of the disrupting process, and looking 

 more like the top of a lava flow. This hard crust of the seolian 

 sandstone has often been described. Alluding to this sandstone, 

 A. Agassiz remarks that it "is " covered with a hard ringing crust 

 when exposed to the action of the sea or the rains." In this con- 

 nection we can here appropriately introduce the fact that it is to 

 his celebrated father, L. Agassiz, that we are indebted for the clue 

 to the nature of the hardening process that affects the surface of 

 the rock. 



The Observations of L. Agassiz on the Salt Key Bank. — 

 But the observations of the elder Agassiz, which were published 

 about half a century ago in the first volume of the Bulletin of the 

 Museum of Comparative Zoology, go further, since they also throw 

 light on the conditions under which the consolidation of the mass 

 of the formation takes place. They were made on the keys or low 

 islands of the Salt Key Bank, which lies between the Great Bahama 

 Bank and the coasts of Florida and Cuba. After describing the 

 oolitic rock (as he terms it) formed by the consolidation of the 

 calcareous sand of the high dunes gathered by the wind, he alludes 

 to its being very hard and to its ringing under the hammer, the 

 weathered surface being implied. But the clue is afforded in his 

 reference to " thin layers of very hard compact limestone, alter- 

 nating with the oolitic beds, which have no doubt been formed in 

 the same manner as the coating of the pot-holes." 



These pot-holes are described as of two kinds, those of recent and 

 those of ancient origin. The first, lying near the water's edge, are 

 " mostly clean excavations," being either empty or containing a 

 little loose sand or pebbles. The second, ordinarily beyond the 

 reach of the tides and the waves, have often been " gradually filled 

 with materials identical with those of the older (oolitic) deposits." 

 They are " generally lined with coatings of solid, compact, and hard 

 limestone, varying from a thin layer to a deposit of several inches 

 in thickness. ... It is plain from their structure that these coatings 



