Page:Africa by Élisée Reclus, Volume 3.djvu/138

106 WEST AFRICA. vessels to seek shelter on the north or leeward coast. The rollers, which break on the beach even in calm weather, and especially from December to April, are perhaps even more formidable than at St. Helena. This magnificent spectacle lasts at times for days and weeks together. But mighty billows 30 feet and upwards in height are sometimes raised within a few minutes and as suddenly stilled. By Evans they are attributed to the fall of enormous icebergs, which break away incessantly from the rocky Antarctic lands, and plunge bodily into the deep. Ascension is of smaller size (30,000 acres) but more regular form than St. Helena, presenting the outlines of a spherical triangle, with its most precipitous side facing towards the trade winds. It culminates in the centre with a lofty cone 2,800 feet high, whence is afforded a comprehensive view of nearly all the now extinct craters, of which geologists have reckoned as many as forty-one. From the central cone Darwin noticed that the mounds of scoriae presented their more sloping side towards the south-east trade winds, while the largest quantities of igneous matter were ejected on the opposite side, where it falls in abrupt escarpments. Most of the craters are cut obliquely by the effect of the aerial current, although nearly all the inner cirques are of extremely regular form. One of them has even received the name of the E-iding School. Volcanic boulders are scattered round the craters, and in the mass of scoriae are embedded some blocks of different formation, such as syenite and granite. But apart from these isolated specimens, the red and calcined mass of Ascension presents nothing but igneous rocks, such as basalts, pumice, pozzolana, or argillaceous clays. Round about the shore-line, however, the masses of broken shells, of corals, and volcanic sands are consolidated into a sort of limestone conglomerate, which may be used as a building material. Certain varieties of this rock acquire the consistency and appearance of white marble, while others are disposed in transparent and almost crystalline layers, covering as with enamel the reefs washed by the tides. This natural cement becomes fixed so rapidly that young turtles hatched in the sands get overtaken and embedded in the concrete mass. Climate. — Flora. When the air remains unrefreshed by the sea-breezes, the temperature becomes very oppressive, for Ascension lies under 7° 57 S. latitude, within 550 miles of the equator. In the roadstead the mean annual temperature is 84° F., which on the breezy uplands falls to 68^^ or even 60"^ F. Although an epidemic of yellow fever carried off a third of the garrison in 1823, the climate is considered exceptionally salubrious, despite the high temperature ; the island has even become a health- resort for Europeans residing on the African coast. The rainfall, much less copious than in St. Helena, is insufficient for the local requirements ; hence the smallest springs, including one discovered by Dampier when shipwrecked here in 1701, are husbanded with the greatest care. The few heavy showers almost entirely disappear in the scoriae covering most of the surface. Since 1860 successful attempts have been made to replant the uplands. The