8 A MONTANE RAIN-FOREST. 



There are no traces of recent volcanic activity in the Jamaican moun- 

 tains and they present to-day the rounded summits and closely set 

 valleys of B typical erosion topography. The underlying rock is 

 mainly readily weathered shale. At the summit of John Crow Peak 

 and at a few localities in the Clyde and Green River valleys there are 

 outcropping* of limestone. In spite of the copious rainfall there are 

 no constant streams above 4,500 feet, but at a very few hundred feet 

 below that elevation the water table emerges to feed numerous swift 

 streams. Owing to the nature of the topography, there are no lakes 

 or ponds, although there are a few depressions on the summit of the 

 Main Ridge itself, which are developed as sphagnum bogs. 



The longer lateral ridges which form the divides between large 

 drainage areas are comparatively gentle in slope (14 to 25). Those 

 ridges which separate smaller drainage areas are steeper (25 to 35). 

 The flanks of these ridges are, of course, steeper still (35 to 45) and 

 in narrow ravines the sides are frequently as steep as 65. Such 

 precipitous slopes, in the absence of resistant rock, are a resultant 

 between the erosive action of the abundant rainfall and run-off and 

 the retaining action of the vegetation. The former of these forces fre- 

 quently overcomes the latter and landslips take place which devastate 

 the vegetation and leave paths which remain unstable and bare for a 

 long time. 



The deepest of the soils is a yellow clay w r hich occurs on ridges and 

 gentle slopes in a few localities in the vicinity of the limestone outcrops, 

 and sometimes attains to a depth of 8 feet. With this exception the 

 soils are shallow and filled w r ith coarse rock fragments. Their humus 

 content is high, but the rapidity of erosion prevents its accumulation. 



The climate of the Blue Mountains is that of all mountainous regions 

 in tropical islands. The temperatures are extremely constant and low 

 as compared with those in the lowlands, although very rarely so low 

 as to make frost possible, and the rainfall is abundant at all seasons. 

 The Blue Mountain Region is, therefore, a tropical montane region, 

 in the terms of Schimper, lying above the hot lowlands and not attain- 

 ing to a sufficient altitude for alpine influences to come into full play. 

 The dominant vegetation is, in accordance with the climate, the ever- 

 green broad-leaved forest, w r hich is here of a type strongly temperate 

 in its floristic make-up and in its vegetative characteristics. 



The economic value of the forests and lands of the Blue Mountain 

 Region is low, as has been hinted in the Introduction. A very small 

 amount of timber is taken out of the forests from time to time to supply 

 the framew r ork for bamboo houses in the neighboring settlements, but 

 the bulk of it stands to-day untouched. Although there are several 

 valuable woods among the mountain trees, notably that of the Podo- 

 carpus, natural obstacles make the forests commercially worthless and 

 they are held as Crown Land for the sake of their value as a cover and 



