502 PROF. J. W. SPENCER ON THE GEOLOGICAL AND [NoV. I9OI, 



XIII. Erosion -Features of the Antigua-Barbtjda Region. 



As before stated, these two islands and connecting banks form a 

 geographical unit, although the greater part is slightly submerged. 

 Whatever erosion had affected the mountain-districts of Antigua, 

 prior to the elevation of the White Limestones, it left but little 

 effect upon the later topography. The physical features of the 

 north-western part of the island are such as show the former 

 uniform thickness of the White Liiuestones from one end of the belt 

 to the other. The surface gently descends in the same direction as 

 the beds are inclined. At an elevation of about 100 feet is a 

 peneplain sloping towards the eastern coast, but out of it rise isolated 

 ridges. Polio wing the trend of the belt, the country is charac- 

 terized by a succession of hills, from 200 to 350 feet above 

 the sea (which in one case only rise to 450 feet), separated by 

 broad undulating depressions reduced to 150 feet above the sea. 

 Thus we find that the atmospheric agents had not merely removed 

 a great thickness of the White Limestones, but dissected the whole 

 region and transformed the incisions into gently-sloping depres- 

 sions, leaving only the higher points as isolated hills, occupying 

 a subordinate portion of the country. One cannot even guess 

 to how enormous an extent these rocks have been removed by 

 degradation. They must have also covered to a greater or less 

 extent the tuffs to the south-west. Such topography indicates 

 the denudation of a region rising only gently above the base- 

 level of erosion (that is, the level of the sea, the margin of which 

 was then located at least beyond the banks) ; for the peninsulas 

 of the eastern coast are the remains, in part, of the White Lime- 

 stones separated by the excavation of the intervening valleys now 

 submerged. 



The configuration of Barbuda conforms to that of the north- 

 eastern portion of Antigua ; and the banks between the islands 

 represent a coastal plain now submerged to 100 or 120 feet, which 

 does not appear to have been subsequently modified by the growth 

 of corals, as in the shallower water immediately about the islands. 



The tufaceous deposits have evidently suffered a greater amount 

 of erosion than the limestones, as the incoherent materials have 

 been removed so as to form the low broad plains of the middle 

 belt of the island, out of which some of the harder rocks, such as 

 the remains of Drew's Hill, rise to elevations similar to that of 

 the limestone -hills to the eastward ; and indeed one eminence 

 at the southern end of the belt (Monk's Hill) shows the remains 

 of this deposit to an altitude of about 700 feet, indicating a differ- 

 ential denudation of nearly this amount. 



The broad valleys of the lower reaches of the streams, with their 

 low gradients, deeply indenting the mountain- zone, appear to 

 belong to the same long period of denudation, but the deeper 

 valleys of the interior of the island have quite different features, 

 suggesting that during the long Miocene-Pliocene Period it was a 

 plateau or mountain-area only partly dissected by the rapidly- 



