26 bulletin: museum of comparative zoology. 



The two foregoing stages of the cockpit topography are always de- 

 veloped in the upper part of the white limestones. Below these is 

 another limestone formation of more even texture, still soluble but less 

 pervious than the former, which forms a temporary resistance plane to 

 the progress of degradation. Still below the latter are the nodular 

 limestones, clays, and conglomerates of the older insoluble and imper- 

 vious formation whose close texture stops the further downward progress 

 of the lime-charged waters which must then find passage in sub-horizontal 

 directions. 



When the bottoms of the cockpits reach the temporary resistance 

 layers of the second group of limestone beds (Fig. 7, in) their peri- 

 meters begin to expand laterally, thereby forming peculiar round or 

 oval valleys with steep sides, commonly known in Jamaica as light- 

 holes (Plate IX. Fig. 2). The downward process continues below the 

 lower limestone, first dissecting it into flat-topped remnants, as shown 

 in Figure 7, iv and v, and finally removing it entirely. 



When the insoluble beds of the older nucleal mountain material are 

 reached, the cockpit topography enters a period of decadence. Down- 

 ward degradation by solution ceases ; the valleys expand by lateral 

 erosion (Fig. 7, vi) and extensive circular enclosed basin valleys result. 

 Furthermore, great springs of water break out at the contact of the 

 limestones and clays, and establish corrasive drainage, absent in the 

 higher limestone districts, which etches the floor of these basins into 

 miniature mountain forms, thus reviving the ancient antecedent topog- 

 raphy. The barriers between these valleys and the coastal drainage 

 are ultimately captured by the latter, and in this manner some of the 

 valleys have become connected with the Coastal Plains of the Liguanea 

 type to be described. Such is the evolution of the topography of the 

 cockpit country and the origin of the interior basin valleys of Jamaica. 



The Interior Basin Vallej/s. — The many basin-shaped depressions 

 occurring throughout the plateau region constitute some of the most 

 interesting i)hysiographic features of the island. Although varying in 

 dimensions, these are all of a uniform type, consisting of deep depres- 

 sions in the summit of the plateau, enclosed by rugged limestone walls, 

 and having a floor established upon the rocks of the Blue Mountain 

 Series, and are covered by accumulations of alluvium or residual soils. 

 The valleys dillcr from one another chiefly in area and irregularities of 

 the relief of the basin bottoms wliich seldom exceed 400 feet in altitude 

 above sea level, while tl>e perimeters of surrounding hills rise from 1,200 

 to 2,500 feet above them, the height varying in diflfereut localities. In 



