ON THE GEOLOGY OF UAIIUADOS. 200 



bottom ; sometimes there is onlj'' one shelf, reaching gradually to 

 10 fathoms with a steep slope to 2/5, sometimes two or even three 

 shelves, especially along the western coast. It would appear, there- 

 fore, that the terrace-like appearance of the raised reefs may be 

 partly due to the original conformation of the reefs before upheaval, 

 and not always or entirely to erosion during pauses in the upward 

 movement. In other words, the outer slopes of the raised reefs 

 may sometimes be the unmodified edges of the reef-plateau, though 

 in many cases, as w^e shall see, the scar^^s exhibit evidence of modi- 

 fication and erosion. 



§ 3. The IIaised Reefs ok Cokal-eocks. 



(a) General Description. — Six-sevenths of the surface-area of 

 Barbados, that is 144 out of 160 square miles, consist of coral-rock 

 or limestone formed largely from the debris of corals. This rock 

 may bo regarded as a sheet or mantle of recentl3''-formed limestone 

 covering and enveloping the greater part of the dome-shaped mass 

 of the older rocks. It must not, however, be viewed as a single 

 even and continuous sheet, for it really consists of a number of 

 separate stages or platforms built up one around the other as the 

 island slowly rose from the sea. Each of these platforms was once 

 a fringing coral-reef like that which now surrounds the greater part 

 of the island, and they now form a succession of steps and terraces 

 of greater or less width, rising one above another from the sea-level 

 to a height of nearly 1100 feet in the centre of the island, each 

 step in the ascent being of slightly older date than the one below. 



The first noticeable feature is that these terraces are not all sub- 

 sidiary to one centre. The principal system of terraces conforms 

 to the central ridge of high elevation which curves round the 

 highest part of the Scotland District between Mount Hillaby and 

 Castle Grant, this system descending by a series of slopes and 

 plateaux from levels of over 1000 feet to a level of about 150 feet 

 on the northern, western, southern, and south-eastern sides. It is 

 limited on the south by the broad valley or depression that extends 

 across the southern part of the island from Bridgetown to the 

 Crane. 



South of this there is a smaller independent system of terraces 

 forming what is known as the Christchurch Ridge. No part of 

 this ridge is more than 400 feet above the sea, and all that portion 

 which is over 300 feet is broken up into a set of irregular east and 

 west ridges; these are concentrically surrounded by a series of 

 lower terraces. 



The newest terraces, or those below 150 feet, having been formed 

 after the union of the two island-centres, are in consequence con- 

 tinuous round the whole western and southern borders of the 

 present island, exhibiting a close parallelism to the present line of 

 coast. 



There is a second ridge at a higher level, which is nearly 

 isolated, and presents some features which are analogous to those 



