1897-98]- LATE FORMATIONS AND GREAT CHANGES OF LEVEL IN JAMAICA. 355 



of vastly smaller proportions than the previous, and is represented by 

 the canyons not yet widened into valleys. The higher mountains are 

 covered with deep soil, the result of the decay of the rocks. This loose 

 material subjected as it is to the heavy tropical rains washing the very 

 steep slopes, suggests that they must have been thrust abnormally high 

 in very recent times, for such material could not long resist the atmos- 

 pheric action. The descent of the mountain streams may be from 500 

 to 1,000 feet in a mile. 



Faulted (?) Basins. — On referring to the map, (Figure 6, page 345), the 

 belt of White Limestones, from 700 to 1,000 feet in altitude above the 

 sea, occurs in front of several deep valleys. The higher portions of 

 these valleys correspond to depressions across the limestones, but the 

 lower portions are too low, and yet so large that they could hardly have 

 been excavated since the canyons have cut through the White Limestone 

 belt, which rises from 200 to 500 feet above the inner valleys. It is 

 suggested that these valleys, although erosion depressions, have been 

 obstructed by recent faultings. The feature is repeated in Cuba, where 

 the writer found the evidence of very late faulting, which brought up 

 the barrier in front of the large Yumuri Valley, near Matanzas. 



St. Thomas-in-the-Vale is a valley about six miles long and from 

 three to five broad, and surrounded by high mountains. At its upper 

 end it has an elevation of 700 or 800 feet, which gradually declines, so 

 that near Bog Walk it is not more than 300 feet above the sea. It is 

 plainly an erosion valley, from which the higher portions could have 

 been drained across the depression in the mountains back of Spanish 

 Town, yet such could not have been the case with the lower level that 

 is now drained by a narrow post-Liguanea canyon. Some have 

 regarded it as having been drained by underground channels. But for 

 such a large valley, this explanation seems somewhat inadequate, and 

 its origin by the uplift of the mountain barrier to the south, accompanied 

 by faulting, is suggested. 



The Terrestrial Oscillations. — Complex as it may seem, there have 

 been numerous oscillations since the early Tertiary days. After the 

 abysmal subsidence of the Pteropod marls, about the early Miocene days, 

 the elevation did not culminate until in the early Pleistocene, when it 

 reached 7,000 to 10,000 feet, as shown by evidence adjacent to the island. 

 The Mio-Pliocene elevation was of moderate porportions, with the Lay- 

 ton submergence, of a few hundred feet, about the close of the Pliocene 

 period. Then the great post-Layton elevation was succeeded by a 

 depression of the land to a few hundred feet lower than now ; another 



