Precambrian time is the whole of geologic time from the very beginning of the earth history until the earliest fossiliferous Cambrian beds were deposited. Precambrian time covers almost 90% of the total length of time that has passed since the formation of the earth. Until recently, however, this long period of geologic time was among the least known segments of the geologic record. The actual absence of fossils in Precambrian rocks makes it very difficult to correlate rocks of one locality to those of the others or to identify the age of geological formations from different localities. By introducing the dating methods based on radioactive decay, reliable age data on minerals and rocks have been accumulated, especially since 1950. Precambrian is now outstanding in availability of a very notable number of exact ages, among which the oldest ones are estimated at 3, 500 million years. The Precambrian rocks are exclusively found in the vast shield areas of the world. In the African continent, they occupy 57% of the whole continent in its areal distribution. In recent years, researches on the Precambrian rocks in the African continent have made a remarkable progress, especially on the following five points: (1) Since the improvement and spread of radiometric dating techniques, various Precambrian orogenic belts and early Palaeozoic one have been dated. Stratigraphic successions of the Precambrian system were greatly revised. Fairly well correlation of Precambrian rocks from one region to another is done throughout the African continent. A. Holmes (1963) wrote vividly this situation of drastic revision of the stratigraphy on Precambrian system in Africa as follows: “For me, probably the most dramatic and unexpected surprise of a decade packed with surprises was the announcement of the great age of the Bushveld Complex, about 2, 000 million years, and the consequent realization that the Transvaal Group of strata must be older still. Until 1901 the Transvaal ‘System’ was correlated on lithological grounds with the Palaeozoic Cape ‘System’. Then for over half a century the Transvaal ‘System’ was confidently thought to be of late Precambrian age and, lithologically, a typical representative of the Algonkian. Yet it has turned out to be immensely older than such characteristically Archean rock sequences as the Grenville of the Canadian shield and Svecofennian of the Baltic shield.” (2) The almost all Precambrian rocks of the African continent have been hitherto considered to represent the Precambrian Craton (Shield). Recently, time, character, and areal distribution of the Precambrian orogenic cycles in Africa have been confirmed, and the following five orogenic cycles have been recognized. a. Upper Luanyi Cycle (more than 3, 000m. y. ago) b. Shamvaian Cycle (2, 700-230m. y. ago) c. Limpopo Cycle (2, 150-1, 650m. y. ago) d. Kibaran Cycle (1, 290-850m. y. ago) e. Katangan Cycle (620-485m. y. ago) The Katangan belt of orogenesis, late Precambrian to early Palaeozoic in age, is shown to be extensively developed throughout Africa. Of recent years, awareness of the significance of this Katangan Cycle has been growing. From an important but essentially local feature, it has grown to the status of a “Pan-African thermo-tectonic episode.” The Kibaran belt of east and central Africa is also probably extended to the Orange River belt and to Natal in South Africa. These Katangan and Kibaran belts represent a distinctive regime of younger orogens consisting of mobile zones which have suffered orogenic deformation from time to time during the past ca. 1, 200m. y., and this younger tructural regime is readily differentiated from older cratons which have remained stable over the past ca. 1, 500m. y. a large part
The author's observations, statistical analyses and review of the previous works lead him to a “problem climate”, which still awaits a satisfactory explanation. That is the weather phenomena particularly of July and August. The author attempted to make a consistent explanation through a geographical approach. Most parts of Uganda and the Kenya Highlands receive the most abundant rainfall in July and August, when the rest of East Africa records practically no rain or the least precipitation in a year (Fig. 3). The ITC theory fails to explain this copious rainfall in these regions, since the ITC is supposed to, be located over Sudan at this time. An attempt was made to explain this by cold fronts invading from the higher latitudes (Henderson 1949). But any kind of migrating system of rain bringing perturbation has not yet been-detected on synoptic weather charts nor by daily rainfall analyses (Thompson 1957 a, 1965, Johnson and Mörth 1960, Johnson 1962). The weather of this time in the Nairobi district is characterized by high, cloudiness (Fig. 4) and frequent drizzles, and yet July and August are the driest months as far as precipitation is conserned. This, too, must be properly explained. Rain in the tropical region is generally considered to be of local character. Indeed, the individual rain are areas small in size (Fig. 2), but there is not infrequently a simultaneous tendency toward increase or decrease of rain areas over an extensive region. The rain in July and Angust is frequently more widespread than local. And the rainy spell may be prolonged for more than a week, or even two weeks (Table 3). All of these phenomena are expected to be associated with upper winds. Fig. 6 shows the seasonal change of the zonal and meridional components of the resultant winds over Nairobi. It is clearly evident that July and August are characteristically under the influence of the westerlies embedded between the SE monsoon and the upper easterlies. The westerlies are observed even over the southern border of Tanzania. Contrary to the commonly held idea that the westerlies come from the Congo region, they are least frequent over the western part of Uganda (Fig. 7) and of Tanzania. The interrelation between the rainfall and the westerlies is further investigated by using the daily aerological and precipitation data for 1964. As a result, it is ascertained that the widespread rains are associated with the westerly incursions. Moreover. the most frequent and the most abundant rainfall during the westerly incursions are found on the west side of Aberdare Mountains (ca. 4, 000 meters), and, to a lesser extent, on the west side of Mt. Kenya (Fig. 8). This rainfall pattern is peculiar to the westerlies season, for the rainiest parts are found on the east facing slopes in other seasons. The season of the westerlies abruptly ended in mid-October in 1964. Accompanied wish this abrupt change in the upper circulation, there occurred a drastic change also in the rainfall pattern. After the alternation of the upper winds, widespread rains became more frequent in the eastern parts of East Africa, that had been dry during the westerlies season. On the contrary, widespread rains gave way to local rains over the Kenya High-lands. Kinematically as well as thermo-dynamically the westerlies favor the formation of clouds. But the existance of the stable upper easterlies over Nairobi suppresses the vertical development of clouds, resulting in stratified clouds and drizzles. On the other hand, the vertical develoment of clouds, which are probably explained by lack of the stable layers aloft, gives rise to heavy showers over the northwestern parts of East Africa. Winds with a component from the pole to the equator have a tendency toward uplift (Flohn 1960). This accounts for the dryness of Tanzania during the westerlies season. The dryness of Northeast Kenya is considered to be accentuated by the lee effect of the westerlies.
Concerning the migration of the cocoa farmers from Akwapim areas to Akim-Abuakuwa areas in the Gold Coast, it has been already pointed out by Miss Polly Hill that this phenomena should be realized as one of the most drastic and capitalistic attitude towards the new cash crop, cocoa. It is, no doubt, very difficult to indicate what is the most important factor affecting to the expansion of the cocoa production. Nevertheless, I think, it is needed to analyze some production effect of the price. From the point of view of the traditional economic structure in the Akwapim areas, most of the male farmers were engaged in the palm and rubber production, and at the same time some of them were traders or craftsmen travelling to Nigeria and Congo Free State. Therefore, they had already contacted directly or indirectly with some capitalistic economy as the traders or craftsmen. According to their techniques of production and natural environments in this areas, they faced with problem whether to expand the traditional cash crops or to introduce the new cash crop at that time. Meanwhile, the export prices of the palm oil falled down drastically from the beginning of 1850's. Therefore, the farmers could introduce and expand cocoa production in place of palm or rubber production without any hesitation. This fact shows us that the Akwapim farmers have already been free from the traditional community lives and have high response to the price-mechanism. I would like to mention that they were now so-called “new farmers”. I have to call them “new farmers” because not only they could substitute palm growing for cocoa production promptly, but also they did expand cocoa production progressively under the migration system.