84 WEST AFEICA. The archipelago also differs from the Canaries and Azores in the generally quiescent state of its volcanic forces. With the exception of Togo, none of the craters have been in eruption since the discovery, and earthquakes are also rare, no violent shocks having been recorded, except in Brava, at the south-west extremity of the semicircle. Iron abounds, especially in the southern group, where an extremely rich titanate of iron occurs on the coasts in the form of black sand, and in such quantities that, when heated by the solar rays, even the Negroes do not venture to tread the ground. Countless cargoes of iron ore might here be shipped. Climate. As in the other Atlantic groups, the mean temperature, equalised by the sur- rounding waters, is less elevated than on the African continent under the same latitude. At the observatory of Praia, in Sara-Thiago, it was 75° F. in 1877, the two extremes in the same year showing a difference of 30° : hottest day, Septem- ber 9th, 91^ F., coldest, December 13th, 61° F. The neighbourhood of the African coast and the influence of the east wind explains this wide range. The climatic conditions are almost exclusively determined by the atmospheric currents, on which depend the heat, moisture, and salubrity of the air. When the north-east trade winds prevail, that is, from October to May, the sky is clear except at sunrise, when the eastern horizon is always overcast. Then follows the wet season, from June to September, during which the land is watered by heavy showers, ** as necessary to the inhabitants as are the waters of the beneficent Nile to the Egyptian fellahin." But the rains and accompanying storms are less regular than on the mainland under the same latitudes, and at times the moisture is insufficient to water the crops, and then the inhabitants are decimated by famine. Sometimes also the north-east trade wind is deflected to the continent, whence it blows over the islands like the blast of a hot furnace. It then takes the name of le^tc, that is, " east wind," which is the harmattan of the Arabs. From the desert this wind brings a large quantity of sand, which is deposited on the islands in the form of impalpable dust. These dust storms may occur at any time, except perhaps in the months of August and September, that is, the season of calms, of variable winds and of heavy showers brought by the sea-breezes. The archipelago lies well within the zone of " dry rains," which extends between 9^ and 16° N. latitude to a distance of 1,200 miles seawards from the African coast. Helmann's observations show that this phenomenon of yellow and red sandy clouds lasts at times several days, and prevails over a space of some 120,000 square miles. To supply such a prodigious quantity of powdered rock extensive mountain ranges must have been worn away during the course of ages, w^hence the present aspect of certain hamadas, or stony wastes, in the Sahara, which for vast spaces offer nothing but smooth polished rock swept clean by the east wind. Some of the dust clouds mingled with animalculac appear to blow with the counter atmospheric currents from South America, but there can be no doubt that the great mass of these sands comes from the African desert.
