Geology. 283 



In this section we appear to have a fair representation of what 

 would he seen in vertical sections through the island at right 

 angles to the coast at any point, the succession heing as follows : 

 (1) A central core of older volcanic and Eocene or Oligocene lime- 

 stones ; (2) heds of basalt, volcanic ash, and thick masses of 

 Orhitoidal limestones enwrapping 1 ; (3) masses of talus, derived 

 mainly from the Miocene rocks and covered by (4) a thick detrital 

 limestone, which is derived from the wear of the reefs which cover 

 the higher portions of the island; (5) a raised reef of much later 

 date, which covers the foot of the cliffs and slopes composed of 4 ; 

 and (6) finally, the late Pleistocene, or even recent, limestones of 

 the sea cliff, which cling to the base of any of the older formations 

 which may be exposed. 



HocJcs of the Central Nucleus in otlur localities. — As far as I know, 

 the only other locality in the island where there is an exposure 

 of the Eocene or Oligocene limestone containing Discocycline 

 Orhitoides is in Sidney's Dale (see Map), a deep gorge on the 

 west coast. This valley is one of several which, beginning at 

 a height of between 400 and 500 feet, runs down to the sea in 

 a generally south-west direction. In some cases (e g. Sidney's 

 Dale) they cut through the sea cliff and terminate at the sea-level 

 (Fig. 7), in others (e.g. Hugh's Dale) they open out on the top of 

 the lowest terrace. These valleys seem to have been originally 

 formed along lines of cracking and faulting ; as a rule, on one side 

 they are shut in by vertical cliffs, on the other by steep slopes. 

 The floor is often formed by basaltic rock, and in one or two 

 instances (e.g. in the lower part of Sidney's Dale Valley) it 

 appears to form an upper edge of a narrow dyke running along 

 the direction of the valley and showing signs of columnar structure 

 at right angles to it (see Fig. 8). In all cases there are signs 

 that in the rainy season a considerable torrent runs down these 

 valleys, and no doubt has contributed to a considerable extent to 

 their present form. I believe that where the sea cliff has been 

 cut through to the sea-level this is entirely the Avork of the 

 running water, and that the movements which first gave rise to 

 these gorges did not affect the lowest terrace, which in fact was 

 formed subsequently to them (see Fig. 7). 



In Sidney's Dale, at about 250 feet above and 25 chains from the 

 sea, the southern wall of the gorge is formed by cliffs varying from 

 60 to 100 feet in height; the northern side is very steep and in 

 places cliff -like, while the total width of the floor is not more than 

 20 or 30 feet, and is mostly occupied by the rounded and water- 

 worn rocks which indicate the existence of a rapid stream at some 

 seasons. When I visited the valley in October it was perfectly 

 dry. The cliffs to the south consist of a basal portion of basalt, 

 about 10 feet high, then a narrow band of soft rock some 18 inches 

 thick, and above this to the summit of the cliff hard yellow 

 Eocene or Oligocene limestone. The basalt has a ground-mass 

 crowded with lath-shaped crystals of plagioclase, some augites, and 



