Vol. 57.] PHYSICAL DEVELOPMENT OF ANTIGUA. 493 



80 long a time by atmospheric agencies that only narrow ridges 

 remain between the valleys and ravines. 



The central portion of the island, with the mountain-belt, as just 

 described, on one side, and bounded on the other by a line of 

 interrupted hills, extending from Dickenson's Bay (near Wetherell 

 Point) to Willoughby Bay, on the southern coast, is characterized 

 by plains at only a slight elevation above the sea, out of which rise 

 isolated hills, among which Drew's has a height of 356 feet. 



The remaining, north-eastern portion of the island forms a third 

 zone, which is marked by undulating plains and hills rising to 

 between 200 and 350 feet above the sea — in one case alone to 

 450 feet. In this portion of the island the drainage is largely 

 absorbed into the porous strata, and is carried off below the surface, 

 leaving it devoid of permanent streams. 



The diverse features of these three belts are dependent upon 

 their geological structure, and their boundaries mark the confines 

 of the principal formations. 



III. The Igneous Basement of the Island. 



The foundation-rocks of the island appear at the surface, in the 

 mountain-belt described above. Their general character, as then 

 understood, was recognized by jN'ugent, and has recently been more 

 fully described by M. Purves.^ He shows that the rocks form a 

 dolerite consisting of triclinic felspars, with little or no magnesian 

 silicate. The phenocrysts are vitreous felspar ; angular grains of 

 magnetite are associated with them. This is an intermediate 

 eruptive, and if included in Palaeozoic rocks would be called a 

 porphyrite, but if in Tertiary rocks an andesite. There is 

 nothing in the character of this rock to establish its age. All that 

 can be said is that it is pre-Tertiary, on account of its underlying 

 the older Tertiary deposits. In places the rock is strongly por- 

 phyritic. Among the hard eruptives occur irregularly beds of 

 fragmental materials or breccias and ashes, which have been 

 more or less subjected to alteration, and these dip north-eastward, 

 passing under the newer formations of the island. The compact 

 trappean beds are often found decomposed, forming a residual soil 

 many feet thick. 



The eruptive formations sufiPered great denudation, both before 

 the succeeding deposition of the Tertiary formations and since the 

 close of the early Miocene Period. The long denudation, under 

 varying conditions since that time, has largely given rise to the 

 physical features of the island with which we are concerned. 

 Such old igneous rocks occur in many of the Windward Islands, 

 but in the ordinary accessible literature they are not generally 

 distinguished from the much later volcanic eruptions which have 

 broken through the older formation. 



' Bull. Mus. Roy. Hist. Nat. Belg. vol. iii (1884-85) p. 279. 



