122 PROCEEDINGS OF THE ACADEMY OF [Feb., 



containing considerable clay. The pure limestone layers are composed 

 of a compact, fine-grained form of calcium carbonate which encloses 

 numerous corals and marine shells; but these, being of the same 

 composition as the ground mass, weather equally with it, and hence 

 do not come out entire by the solution of the stone, but only appear 

 on the surface in section. 



The layers of marly limestone upon weathering break up into 

 blocks of varying size, and tops of hills formed of these layers are 

 covered with loose, detached blocks of all sizes. An examination of 

 any of these boulders shows at once that they are very porous and 

 loosely held together; when they disintegrate, the rock passes into a 

 coarse gravel, composed of small angular fragments, and finally into 

 smaller fragments and clay. The residual clay from the marly rock 

 is white to buff-colored, or sometimes yellow-ochre color. This rock, 

 on hill-tops, weathers deeply, by boulder weathering; its porous 

 character makes the soil drain quickly after rain, and such hill-tops 

 are notably dry. They are generally flat-topped, if the hill is of 

 considerable size, and are covered with a growth of the "broom-palm," 

 the leaves and debris from which cover the ground to the depth of 

 several inches. This coating of dead leaves on the ground is generally 

 so dry that the snails find little to feed on in it, and these dry hill-tops 

 furnish few specimens of land gasteropods. 



The purer limestone layers, on the other hand, do not weather into 

 blocks by boulder weathering, but the rock passes at once into a fine 

 clay, which is generally very dark red to reddish-brown, as Hill 

 describes that of the Cobre formation. This residual clay, of course, 

 does not persist on the exposed surfaces of rock on the hill-tops, but 

 is washed off by the rains into the hollows and valleys. This pure 

 limestone presents bare rock surfaces that are not porous, and do not 

 absorb moisture; they are not smooth, however, but are honeycombed 

 with irregular holes and cavities, some of large size, and these are, 

 in miniature, the structures characteristic of the Cockpit country. 

 Thus vertical or inclined cylindrical holes or pits may be seen in this 

 limestone, ranging from two or three inches to ten feet in diameter, 

 and often as many feet in depth. The surface of the rock is rough 

 and jagged, sharp spires and knife-edges are characteristic of this 

 pure limestone on the hill-tops. The rock is not porous, but the 

 holes and pits retain the moisture; and the limestone is such a good 

 conductor that moisture condenses and remains on the surface after 

 the deposition of dew each night. These limestones are the most 

 favorable places for land snails. In the country about Mandeville 



