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, 111) 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, 1v 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, vr) 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 Valleys. — The many basin-shaped depressions 
occurring throughout the plateau region constitute some of the most 
interesting physiographic 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 differ from one another chiefly in area and irregularities of 
the relief of the basin bottoms which seldom exceed 400 feet in altitude 
above sea level, while the perimeters of surrounding hills rise from 1,200 
to 2,500 feet above them, the height varying in different localities. In 
