Page:EB1911 - Volume 06.djvu/938

 as pebbles in conglomerate-beds. They are ground to pieces by friction against harder masses and help to form the matrix. The size of the pebbles varies greatly; occasionally they are 10 or 20 ft. in diameter, more frequently they are a foot or less. The cementing matrix in which the pebbles are embedded usually bears some resemblance in composition to the nature of the pebbles, but contains a larger proportion of the softer ingredients, such as clay, mica, weathered felspar, calcite and dolomite. Often it resembles a felspathic or calcareous sandstone; if limestone fragments are common it may be highly calcareous, or may be in large measure dolomitic. Often the matrix is stained red by compounds of iron. The “brockram” of the north of England is a well-known Permian limestone-conglomerate. The Dolomitic Conglomerate is a similar rock, but of Triassic age. Both of these are often extensively dolomitized and pass into breccias, where their fragments are angular and unworn. The pebble beds of the Bunter (Triassic period) are also familiar to geologists. They cover extensive areas in the midlands of England, and are well seen at Budleigh Salterton on the south coast. The pebbles are mostly quartzite with granite, chert, sandstone and igneous rocks.

Conglomerates are rarely well bedded, showing at most a rude stratification, but they may contain intercalations of finer materials such as sandstone and shale, which indicate the bedding clearly. In these fossils may be found, but they do not often occur in the conglomerates themselves, as the conditions are generally unsuitable for the preservation of organic remains. The pebbles, however, may be highly fossiliferous, and sometimes important evidence is provided by this means as to the age of the conglomerate. On account of the imperfect stratification it is often very hard to estimate the thickness of conglomerates, and this difficulty is increased by the fact that many of them must have been laid down as sloping banks of pebbles and not as flat layers of deposit. Conglomerates are merely consolidated gravels, and have originated mostly on seashores or in shallow waters near land. They are typical shore formations, and are especially frequent where one series of stratified rocks rest upon an older group unconformably. Other conglomerates occur along with fine-grained red sandstones, salt beds and such rocks as accompany desert deposits. We may compare them with the accumulations of pebbles which cover extensive areas of existing deserts. A quite distinct group of conglomerates characterizes regions where the rocks are much broken and sheared; these may very closely resemble true conglomerates, but have really been produced by the mashing together of rock masses along zones of fracture and movement. They are known as “crush-conglomerates” or “auto-clastic rocks.” Conglomerates may undergo metamorphism, and are then converted into “conglomerate-gneiss” or “conglomerate-schist.” Their pebbles are flattened and dragged out of shape by interstitial movement, while the matrix becomes highly crystalline. One of the best-known examples of this is the Obermittweida gneiss (Saxony).

 CONGO, formerly known as Zaire, the largest of the rivers of Africa, exceeded in size among the rivers in the world by the Amazon only. The Congo, though it has a shorter course than the Nile, has a length of fully 3000 m. and a drainage area estimated at 1,425,000 sq. m., with a diameter of some 1400 m. either way. This vast area includes the equatorial basin of Central Africa and much of the surrounding plateaus. West and north the Congo basin is bounded by comparatively narrow bands of higher ground, while east and south the drainage area of the river includes considerable portions of the high plateaus of east and south Central Africa. The main drainage of the Congo system is thus north and west, and these two directions dominate the great bow-like sweep of the main stream before it is deflected south on approaching the western highlands, through which it finally forces a way to the Atlantic Ocean. From the high lands of the south and east in which the head-streams of the Congo have their origin, the land falls in a succession of steps, generally marked by gorges or rapids in the upper courses of the streams. Besides the main stream most of the affluents of the river are navigable for considerable distances; in all there are over 6000 m. of navigable water in the Congo basin and 20,000 m. of overhanging wooded banks. On the Congo alone are over 4000 islands, many of considerable length—some fifty of them are over ten miles long. The volume of water poured into the Atlantic is at least 1,200,000 cubic ft. per second.

Head-Streams.—The most distant head-streams of the Congo are far to the north and east of those most to the south, and it is difficult to determine which stream is the “parent” river. The easterly head-streams are, however, regarded generally as marking the true course of the Congo. The most remote of these rivers is the Chambezi, which, with its tributaries, rises (in British territory) on the southern slope of the plateau between lakes Nyasa and Tanganyika at an elevation of about 6000 ft. The watershed is formed by the crest of the plateau, and is perfectly distinguishable, save at a spot called Ikomba, about half-way between the lakes, where is a swamp which drains to both the Atlantic and the Indian oceans. The Chambezi source is in 9° 6′ S., 31° 20′ E. Its chief tributary, the Karungu, rises in 9° 50′ S., 33° 2′ E. The Chozi, an affluent of the Karungu, rises in the same latitude as the Chambezi, about half a degree to the east of that stream. After the junction of the Karungu and Chambezi the river flows in a south-westerly direction through a fairly fertile country, and receiving many tributaries becomes a large river with steep wooded banks and many islands. Its width varies greatly, from 30 yds. to 2 m. in a comparatively short distance; its depth is rarely less than 14 ft. In its lower course the Chambezi passes through papyrus marshes, and dividing into several channels, enters the vast swamp which forms the southern part of Lake (q.v.). The large river, known as the Luapula (Great River), which issues from Bangweulu in 11° 31′ S. and runs south through this swamp, may be regarded as a continuation of the Chambezi, there being a channel from the one stream to the other. The Luapula on leaving the swamp bends west and then south—reaching 12° 25′ S.—and approaches the watershed of the Zambezi, receiving several southern tributaries. The source of its most southern affluent, and therefore the most southern point in the Congo basin, is approximately in 13° 30′ S. Turning north the Luapula precipitates itself down the Mumbatuta (or Mambirima) Falls (12° 17′ S., 29° 15′ E.), the thunder of which can be heard on a still night for 8 or 9 m. The river, the width of which varies from 250 to 1200 yds., is almost unnavigable until below the Johnston Falls (Mambilima of the natives), a series of rapids extending from 11° 10′ to 10° 30′ S. Below the falls the river is navigable by steamer all the way to Lake Mweru—a distance of 100 m. Before entering (q.v.) the Luapula again passes through a swampy region of deltaic character, a great part of the water escaping eastwards by various channels, and after spreading over a wide area finally passing into Mweru by lagoon-like channels east of the main Luapula mouth. From Bangweulu to Mweru the fall of the river in a total distance of 350 m. is about 700 ft. The river (known now as the Luvua) makes its exit at the N.W. corner of the lake, and bending westwards in a winding course, passes, with many rapids, across the zone of the Kebara and Mugila mountains, falling during this interval nearly 1000 ft. In about 6° 45′ S., 26° 50′ E. it joins the Kamolondo (otherwise Lualaba), the western main branch of the Congo, which, as it flows in a broad level valley at a lower level than the eastern branch, is held by some to be the true head-stream. The Kamolondo is formed by the junction of several streams having their source on the northern slope of the south-central plateau as it dips towards the equatorial basin. This escarpment contains many heights exceeding 6000 ft. The streams flowing south from it belong to the Zambezi basin, but the watershed is not everywhere clearly defined. Thus the Lumpemba (an affluent of the Lokuleshe, one of the main tributaries of the Lubudi) rises in 11° 24′ S., 24° 28′ E., 3 m. S. 