Vol. 57.] PHISICAI DETELOfllENT OF AKGUltLA, ETC. 631 



features are characteristic of denudation at the base-level of 

 erosion, since modified by atmospheric and wave-action during the 

 changes of level of the land. The broad valley-like depressions, 

 extending from the precipitous slopes which bound the submarine 

 plateau on all sides, except that on the Atlantic, to the neighbouring 

 submerged tablelands, have also the features of base-levels of 

 erosion, although they are now from 2000 to 2500 feet below the 

 surface of the sea. As has been seen, the region, whether larger 

 or not, was a land-area during the long Miocene-Pliocene Period. So 

 also the degradation, during this time, not only carried away most 

 of the White Limestone Series, but also removed the very durable 

 siliceous limestones of St. Martin and formed great valleys which 

 have so encroached upon the districts of each other as to reduce 

 the mountain-ranges to isolated ranges separated by comparatively 

 low divides, in which the evidence of differential erosion, in excess 

 of that upon the hills, amounts to 500 feet or more of the hard 

 siliceous limestones alone. Furthermore, there is no evidence of 

 late local elevation of the St. Martin mass above the floor 

 of the drowned Antillean chain, as in the mountainous part of 

 Guadeloupe, where there has been an uplift, which was confined 

 to the district of the more recent volcanic activity, but did not 

 extend to the limestone-area of even that island. As the same 

 phenomena occur in other islands, it can scarcely be doubted 

 that the coastal plain extended to Antigua-Barbuda, St. Kilts 

 (now mostly a mountain-range), the Saba Banks (a plain of 100 

 square miles in area, covered with 75 to 150 feet of water), and 

 Sombrero. In this case, the phenomena indicate an elevation of 

 3000 feet above the present height, when the broad valleys between 

 the drowned plateaux were being fashioned at the base-level of 

 erosion, with the present submarine banks standing out as 

 prominent tablelands at a period subsequent to the early Miocene 

 emergence. With the subsequent sinking of the land later in the 

 Miocene-Pliocene Period, the surface of the late tablelands was 

 reduced to a low altitude, so as to permit of the atmospheric 

 moulding of the surface of the (now mostly sunken) banks into 

 forms characteristic of those of the base-level of erosion, with the 

 margins of the mass modified by ocean- waves during the changes 

 of level. 



After the sinking of the area, subsequent to the existence of the 

 extensive land-surfaces of the Miocene-Pliocene Period, when the 

 undulating topography described was formed, a further subsidence 

 carried it down to perhaps 200 feet below the present level, 

 leaving only insignificant portions of the islands at about the close 

 of the Pliocene Period, and giving rise to the accumulation of beds 

 containing only a now living fauna. 



Then followed the period of great elevation, having a duration 

 , very much shorter than that of the Miocene-Pliocene Period, with a 

 tremendous amount of denudation, carving deep valleys out of the 

 margin of the tableland. The excessive erosion was noted by 

 Mr. Cleve in St. Bartholomew, where the surface is strewn with 

 blocks and boulders. Yast quantities of boulders of decomposition 



