Page:EB1911 - Volume 27.djvu/203

 matrix consolidated by the deposition of secondary silica. The conglomerate bands and quartzites contain large quantities of iron pyrites deposited subsequent to their formation, that in the conglomerates containing the gold. Sericite in the form of scales and films characterizes those portions which have been faulted, squeezed or sheared. Sheets of diabase, apparently volcanic flows, and numerous dykes interfere with the regularity of the stratification. The theory of the subsequent infiltration of the gold is that generally accepted. No fossils have been discovered, and except that they represent some portion or portions of rocks of the Pre-Cape formation the age of the upper Witwatersrand beds, as well as that of the lower division, remains an open question. They may safely be considered to be among the oldest auriferous sediments of the world.

Vaal River System.—This consists largely of rocks of igneous origin, of which the amygdaloidal diabase of Klipriversberg forms the type. The other rocks include igneous breccias, shales, coarse conglomerates and grits. Near Reitzburg the coarse conglomerates reach a thickness of 400 ft. and about 500 ft. at Kroomdraai. This system rests unconformably on the Witwatersrand series and is unconformably overlain by the Transvaal system. It must, however, be acknowledged that these relationships are very imperfectly understood. Compared with other formations they occupy restricted areas, being only met with south of Johannesburg, around Wolmaransstad, Lichtenburg and east of Marico.

Transvaal System.—This is a very definite sequence of rocks covering immense areas in the centre of the country. The following groups are recognized: Waterberg Series, Pretoria Series, Dolomite Series, Black Reef Series.

The Black Reef Series is composed of quartzites, sandstone, slates and conglomerate. It varies in thickness from 100 ft. in the southern Transvaal to 1000 ft. at Lydenburg. Thin bands of conglomerate, sometimes auriferous, occur near the base.

The Dolomite Series, known to the Dutch as “Olifants Klip,” consists of a bluish-grey magnesian limestone with bands of chert. The thickness varies from 2600 ft. in the Witwatersrand area to 5000 ft. around Pretoria; and is about 2600 ft. about Lydenburg. It is worn by solution into caves and swallow-holes (Wondergarten). Gold, lead, copper and iron ores occur as veins. So far it has proved to be unfossiliferous. Dykes and intrusive rocks are common.

The Pretoria Series, formerly known as the Gatsrand series, consists of repeated alternations of flagstones and quartzites, shales and sheets of diabase. These follow conforrnably on the Dolomite series. In the Marico district the shales become highly ferruginous and resemble the Hospital Hill slates of the Witwatersrand series. Near Pretoria duplications of the beds, due to over-thrusting, are not uncommon.

The Waterberg Series lies unconformably on the Pretoria series. The colour is usually red, forcibly recalling the Old Red Sandstone and Trias of England. Sandstones, quartzites, conglomerates and breccia make up the formation. They occur to the north-east of Pretoria and occupy still wide areas in the Waterberg district.

A complex of igneous rocks of different ages covers immense areas in the central Transvaal. Various types of granite are the predominant variety. Syenites, gabbros, norites and volcanic rocks are also represented. The granite contains two varieties. One is a red granite intruded subsequently to the Waterberg sandstones; another is a grey variety considered to be older than the Black Reef series and possibly older than the Witwatersrand series.

The Karroo System attains its chief development in the south-eastern Transvaal in the districts of Ermelo, Standerton and Wakkerstroom.

The latest classification of Molengraaff subdivides the beds as follows:—

The Dwyka conglomerate resembles the same bed in the Cape province. The boulders consist of very various rocks often of large size. Many of them show glacial striae. The direction of striae on the underlying quartzitic rocks, particularly well seen near the Douglas colliery, Balmoral, point to an ice movement from the north-north-west to south-south-east.

The Ecca series, as in the Cape, consists of sandstones and shales. Seams of coal lie near the base, some of them exceeding 20 ft. in thickness, but in this case layers of shaly coal are included. The overlying sandstones afford good building stones, and frequently, as at Vereeniging, yield many fossil plants. These include among others, Glossopteris browniana, Gangamopteris cyclopteroides, Sigillaria Brardi, Bothrodendron Leslii, Noeggerathiopsis Hislopi.

The Karroo beds lie almost horizontally, in marked contrast to the highly inclined older rocks. Their distribution, other than in the south-eastern districts, is imperfectly understood. Remnants have been found of their former existence in the neighbourhood of Pretoria; and portions of the Bushveld Sandstone have recently been relegated to the Karroo formation.

The diamond pipes probably represent some of the most recent rocks of the Transvaal. They may be of Cretaceous age or even later, and in any case belong to the same class as those of Kimberley. The recent deposits of the Transvaal may be considered to be insignificant. They include the gravels and alluviums of the present streams and the almost ubiquitous red sand of aeolian origin.

Climate.—Although lying on the border of and partly within the tropics, the Transvaal, owing to its high general elevation, and to the absence of extensive marshy tracts, enjoys on the whole a healthy invigorating climate, well suited to the European constitution. The climate of the high veld is indeed one of the finest in the world. The air is unusually dry, owing to the proximity of the Kalahari Desert on the west and to the interception on the east by the Drakensberg of the moisture bearing clouds from the Indian Ocean. The range of temperature is often considerable—in winter it varies from about 100° F. in the shade at 1 p.m. to freezing point at night. During summer (Oct.–April) the mean temperature is about 73°; during winter about 53°. Nov.–Jan. are the hottest and June–July the coldest months. The chief characteristic of the rainfall is its frequent intensity and short duration. During May to August there is practically no rain, and in early summer (Sept.–Dec.) the rainfall is often very light. The heaviest rain is experienced between January and April and is usually accompanied by severe thunderstorms. On the eastern escarpment of the Drakensberg the rainfall is heavy, 50 or 60 in. in the year, but it diminishes rapidly towards the centre of the plateau where it averages, at Johannesburg about 30 in., while in the extreme west as the Kalahari is approached it sinks to about 12 in. The winds in winter are uniformly dry while dust storms are frequent at all seasons—a fact which renders the country unsuitable for persons suffering from chest complaints. In the eastern part of the plateau snow occasionally falls, and frost at night is common during winter.

The banken veld district is also generally healthy though hotter than the plateaus, and malarial fever prevails in the lower valleys. Malarial fever is also prevalent throughout the low veld, but above 3000 ft. is usually of a mild type. Nearly all the country below that elevation is unsuitable for colonization by whites, while the Limpopo flats and other low tracts, including the district between the Drakensberg and the Lebombo hills are extremely unhealthy, blackwater fever being endemic. In the low veld the shade temperature in summer rises to 113° F., but the nights are generally cool, and down to 2000 ft. frost occurs in winter. The rainfall in the low country is more erratic than on the plateau, and in some districts a whole year will pass without rain.

Flora.—The general characteristic of the flora is the prevalence of herbaceous over forest growths; the high veld is covered by short sweet grasses of excellent quality for pasturage; grass is mingled with protea scrub in the middle veld; the banken veld has a richer flora, the valley levels are well wooded, scattered timber trees clothe their sides and the hills are covered with aloe, euphoria, protea and other scrub growths. Among the timber trees of this region is the bolkenhout of terblanz (Faurea Saligna) which yields a fine wood resembling mahogany. The scrub which covers the low veld consists mainly of gnarled stunted thorns with fattened umbrella shaped crowns, most of the species belonging to the sub-order mimoseae. A rare species is the acacia erioloba Rameel doorn, akin to the acacia giraffae of Bechuanaland. The wild seringa (Burkea africana) is also characteristic of the low veld and extends up the slopes of the plateau. The meroola (sclerocarya caffra) a medium sized deciduous tree with a rounded spreading top is found in the low veld and up the slopes to a height of 4500 ft. It is common in the lower slopes of the rands of the low veld. Cotton and cotton-like plants and vines are also native to the low veld. Few of the low veld bushes are large or straight enough to furnish any useful wood, and timber trees are wholly absent from the level country. The forest patches are confined to the deep kloofs of the mountains, to the valleys of the larger rivers and to the sea slopes of the Drakensberg and other ranges, where they flourish in regions exposed to the sea mists. These patches, called “woodbushes,” contain many hardwood trees of great size, their flora and fauna being altogether different from that immediately outside the wood. Common species in the woodbush are three varieties of yellow wood (Podocarpus), often growing to an enormous size, the Cape beech (myrsine), several varieties of the wild pear (Olinia) and of stink wood (Oreodaphne) ironwood and ebony. The largest forest areas are in the Pongola district and the Haenertsburg and