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Show 1 9 0 5 . ] CAPE VERDE MARINE FAUNA. 1 8 5 shallow beds of limestone which clothe the lower slopes of the sides of these two valleys, and this is deposited as the cement of the sandstone when the fresh water mixes with the salt. But for the interference of tides and waves the level of this rock-bed would be that of the valley, i. e. the top of the sand-beach, where indeed a very soft rock does occur. But the sea lias cut down this mass to the level determined by the height of the tides etc. in the usual way. As in the case of the reefs of Zanzibar, we are here shown how purely physical causes, aided by protective organic growths, can produce reefs closely resembling those the mass of which is due to growth of organisms in silu, which leads to the conclusion that the forms .characteristic of true coral-reefs are very largely due to the physical action of the sea as well as to the laws of growth of the organisms themselves. That the formation of this sandstone-rock is still proceeding seems most probable, and it would be interesting to know definitely the conditions of the landward side of the strata underneath the alluvium of the valley. The solvent and eroding actions of the sea are very nearly balanced by the growth of calcareous organisms, but the presence of outlying rocks and submerged masses indicate that the sea is slowly encroaching. It is probable that cementing of fresh material is being carried out on the landward side of the strata at the point where fresh and salt water meet-a point which is moved backwards just as fast as the sea encroaches. It is to be noted that the occurrence of beach-sandstone is not confined to valley-mouths. Smaller deposits occur elsewhere, and a sample I collected at the north point of St. Antonio has the appearance on the surface and the extreme hardness characteristic of the East-African " Coral-rag." 6 . S u m m a r y a n d C o n c l u s io n s . 1. Faunistic.-Although the Indo-Pacific Oceans are one faun-istic area, there is no fauna common to the tropical seas of the world, so far as the evidence of the Cape Verde Islands goes, though certain species are common to the Tropics of both Atlantic and Indian Oceans. If the Tropical Atlantic is a distinct area characterised by a special fauna, its northern limit is carried far to the south by cold currents (and probably northwards in the south), as in spite of the position of these Islands, between 17° and 15° N.*, their fauna has a considerable constituent derived from the subtropical zone. But it may later appear that this portion of the fauna is unduly conspicuous through its species being already well-known, while the constituent derived from the Tropical Region, if any, will not be known until the systematic examination of the collections is completed. The scanty representation of some groups {e. g. Corals and * Compare the fauna of Suez Bay, which is tropical, though situated in latitude 30° N. |
