504 PROF. J. W. SPENCER OST THE GEOLOGICAL AND [Nov. IQOT, 



The second condition is the result of atmospheric denudation and 

 rapidly-descending streams, producing deep valleys, headed by 

 amphitheatres, which are constantly receding into the highlands, 

 and cause the shallow channels upon the surface to become eventually 

 transformed into canons and valleys dissecting the plateau. 



The difference between the erosion-features of the Miocene- 

 Pliocene and the early Pleistocene Periods (separated by the Friar's 

 Hill submergence) lies in the low undulating topographic forms of 

 the first-mentioned, requiring a long period of development at a low 

 elevation ; and the deep valleys and sharp outlines of the latter, 

 produced rapidly at considerable heights above the drainage-level 

 of the region. But it should not be forgotten that the valleys on 

 the high tablelands, well within their borders and not affected 

 by the declivity of their margins, may still preserve the flattened 

 features of a lower level of erosion. 



The erosion-features of the higher parts of the mountains of 

 Antigua— highlands dissected into narrow ridges and deep valleys 

 of o-reat declivity — are the result of the older degradation continued 

 to lihe present time without interruption by submergence. ^ This 

 complete dissection of the mountain-districts is an illustration of 

 the effects which would have been produced upon the Antigua- 

 Barbuda plateau had the secondary period of denudation under 

 conditions of great elevation been continued sufficiently long. 



A considerable portion of the Friar's Hill Series was removed 

 during the early Pleistocene elevation. Again there has been a 

 certain amount of denudation since the epoch of the Cassada-Garden 

 Gravels, and also since the accumulation of the low marls on the 

 eastern coast, this latter extending below sea-level. 



The relative amount of work during the last episode is not so 

 well defined in Antigua as in other islands, and deserves further 

 study in the field. 



XIY. Summary and Conclusions as to Changes of Level ov 



Land and Sea. 



As has already been shown, this region was an extensive land- 

 surface during the Miocene-Pliocene Period, which was eventually 

 reduced to a comparatively low elevation above sea-level before 

 the close of that time. This is the conclusion arrived at from the 

 evidence within the boundaries of the submerged plateau, sur- 

 mounted bv the islands of Antigua and Barbuda. But outside 

 the limit of this paper there is evidence that leads me to think 

 that during the earlier part of the period the whole region was at 

 least 2000 feet higher than now, when the broad valleys between 

 the Antiguan mass and the neighbouring islands on the submarine 

 Antillean plateau were being fashioned out of a higher tableland : 

 of this a remnant remains in the Antigua- Barbuda plateau, the 

 surface of which was reduced to its present form in the latter 

 part of the period. I have not observed in Antigua the evidence 

 of an insular elevation, as seen in the islands where late volcanic 



