632 PROP. s. w. si^EiTcER oif TfiE clEOtOGtcAi AnD [!N"ov. tpoi, 



have been left upon the surface of St. Martin, on account of the finer 

 materials of the decomposed igneous formation having been washed 

 away. So also most of the mechanical deposits, supposed to have 

 been accumulated during the subsidence at the close of the Pliocene 

 Period, have been removed by the great amount of subsequent 

 erosion. But the epoch of deep valley-making did not last 

 sufficiently long for the complete dissection of the tablelands. 

 One such, however, is seen at the south-eastern extension of the 

 sunken platform, which is now from 150 to 200 feet below the 

 sea-level, with a channel 2 or 3 miles wide extending across it 

 and excavated to a depth of more than 1146 feet below sea-level; 

 thus separating a small submarine plateau from the main mass — 

 an illustration of how the drowned tablelands are detached. This 

 is an actual development of the dissection of plateaus. But the 

 valleys indenting the submarine plateau usually head in cirques 

 or amphitheatres. Two conspicuous examples of these occur west 

 of Anguilla, of much the same depth. One of them is a con- 

 tinuation of Crocus Bay. It extends 10 miles within the border of 

 the mass where its surface-plains are covered by less than 100 feet 

 of water, but the valley at its head is more than 680 feet deeper. 

 The watercourses upon the summit of the tableland, and within 

 its margin, were also somewhat deepened. Thus that at Philipsburg 

 was deepened to the extent of 60 feet or more beneath the now 

 silted-up basin forming the salt-ponds. The excavations, whose 

 great depths, well within the borders of the submerged lands, 

 extend to the broad valleys of the great Antillean plateau, which 

 in this vicinity are sunk to 2500 or 3000 feet, consequently 

 indicate that the land stood at such an altitude when they were 

 formed. But the data for determining the additional height to 

 which the now drowned Antillean plateau itself was raised are not 

 obtained here. It was during this period of elevation that the 

 caverns of St. Martin appear to have been excavated to below sea- 

 level. 



Sombrero, already left as an isolated promontory at the close of 

 the Miocene-Pliocene Period, must have suffered an incalculable 

 amount of destruction from its re-elevated summit precipitating the 

 moisture from the trade- winds. 



This epoch of great elevation was subsequent to the deposition 

 of the marly beds containing the modern fauna, considered as 

 occurring at the close of the Pliocene Period, and consequently 

 after the Lafayette epoch of the American continent, or in the 

 early Pleistocene Period. 



The following subsidence of 200 feet below the present level, 

 with the deposition of the St. Martin gravels and boulder-pavement, 

 submerged Anguilla, and left only a few rock-ridges standing out 

 in the ocean. This was a mid-Pleistocene feature, nearly con- 

 temporary with the depositions of the Cassada-Garden Gravels of 

 Antigua, the Lower Petit-Bourg Series of Guadeloupe, or the 

 Columbia formation of the American continent. 



The subsequent elevation of the land to a height of 150 or 180 



