1191.] NATURAL SCIENCES OF PHILADELPHIA. 123 



the marly limestone often forms the hill-top, and the purer limestone 

 is found at a lower level ; and this is true along the hills to the north- 

 west also, towards Somerset and Green Vale, but the crest of the still 

 higher hill, fronting the plain of St. Elizabeth, is mainly composed of 

 the pure limestone. The marly rock, weathering more slowly, 

 would form the hill-tops in the more or less uneven plateau in which 

 Mandeville is located, but along the escarpment edge of this plateau 

 to the west the purer limestone can form the ridge. While there are 

 certainly several different alternating layers of these two types of 

 rock, the general observation holds true that wherever in the island 

 the purer, honeycombed, jagged limestone is encountered, the land 

 molluscs are more plentiful and larger. 



When Jamaica was untouched by cultivation, there is little doubt 

 that all the upland limestone district, and in fact all the surface, 

 except the swamps and savannas, was covered with a continuous 

 forest. Cliffs of rock and exposures of bare rock on some hills were 

 the only exceptions. Under such conditions free migration of the 

 land molluscs from one part of the island to another was possible. 

 In the Mandeville region, where there is no surface water, but the 

 topography is characterized by cup-shaped valleys enclosed by hills, 

 often with no possible connection for surface flow of water from one 

 valley to another, the entire surface was forested before the introduction 

 of cultivation, some 50 years ago. In many places virgin forest is 

 still to be seen on hill-tops, and sometimes extending down into 

 valleys and gullies and connecting hill with hill. The land that is 

 first cleared is in the small cup-shaped valleys, where the soil and 

 humus washed from the wooded hills collect; as the area of cultiva- 

 tion extends, it involves the slopes of the hills and the cultivated or 

 cleared areas join, but they have islands of virgin forest, in the form 

 of the rocky hill-tops, which are not only not arable, but the clearing 

 of which would be a positive detriment to the estates. This is for- 

 tunate, for in these islands the plants and animals find a refuge from 

 the encroaching "cultivations." Many of these woods have existed 

 in their present state since the first clearing of the land, and are only 

 occasionally entered now for the cutting of a little firewood or an 

 occasional hardwood tree for lumber, and there is every reason to 

 believe that they will continue to exist unchanged for generations. 

 Each one of these isolated patches of woodland becomes a definite 

 colony fOr the land molluscs, many of which colonies have existed 

 in their present state for periods varying from 20 to 70 or 80 years. 

 In these areas are found not only the present living snails, but the 



