In Egypt we find the alluvial soil a scarcely less interesting object of study than the rocks upon which it rests. These are limestone, sandstone, and granite, the latter of which, in Upper Egypt, often rises 1,000 feet above the level of the Nile. Not many years ago were discovered about 100 miles east of the Nile, and in 28 deg. 4 min. of north latitude the splendid ruins of the ancient Alabastropolis, which once derived wealth from its quarries of alabaster. Farther south are the ancient quarries of jasper, porphyry, and verd antique. The emerald mines of Zebarah lay near the Red Sea.
The Atlas range in Algeria is better known than elsewhere. It is as described above, but at Calle there are distinct traces of ancient volcanoes. Iron, copper, gypsum, and lead are found in considerable quantities. Cinnabar is found in small quantities. Salt and thermal springs abound in many parts of Algeria, amethysts in Morocco, slates in Senegambia, and iron in Liberia, Guinea, the Desert of Sahara and many other parts of Africa.
Gold, gold-dust, and iron are among the best known of the mineral riches of Africa, and are the most generally diffused throughout the continent. In the country of Bambouk, in Senegambia, most of the gold which finds its way to the west coast is found. Here the mines are open to all, and are worked by natives who live in villages. The richest gold mine of Bambouk, and the richest, it is believed, yet discovered in Africa, is that of Natakoo—an isolated hill, some 300 feet high and 3,000 feet in circumference, the soil of which contains gold in the shape of lumps, grains, and spangles, every cubic foot being loaded, it is said, with the precious metal. The auriferous earth is first met with about four feet from the surface, becoming more abundant with increase of depth. In searching for gold the natives have perforated the hill in all directions with pits some six feet in diameter and forty or fifty feet deep. At a depth of twenty feet from the surface lumps of pure gold of from two to ten grains weight are found. There are other mines in this portion of Africa, gold having been found distributed over a surface of 1,200 square miles. The precious metal is not only found in hills, the most of which are composed of soft argillaceous earth, but in the beds of rivers and smaller streams, so that the lines of Bishop Heber’s well-known missionary hymn are truthful as well as poetical:—
“Where Afric’s sunny fountains,
Roll down their golden sands.”
The gold mines of Semayla, which are some forty or fifty miles northward of those of Natakoo, though nearly as rich as the latter, are in hills of rock and sandstone, which substances are pounded in mortars that the gold may be extracted. Barth judged that gold would be found in the Benue river, the principal eastern tributary of the Niger. Gold, silver, iron, lead and sulphur have been found in large quantities, and were long profitably mined in the mountainous districts of Angola. In Upper Guinea gold and iron are deposited in granitic or schistose rocks. The interior contains vast quantities of iron which might be easily mined, but the natives are not sufficiently enterprising to accomplish much in this respect. Gold is also obtained in the beds of some of the rivers of Guinea. In Mozambique, on the east coast, the Portuguese have for a great length of time had a considerable commerce in gold obtained from mines near the Zambezi, in the region near the western limit of that province. It has already been stated that here Dr. Livingstone discovered deposits of coal. Along the Orange and Vaal rivers, in extreme South Africa, have recently been discovered diamond fields which some noted scientists believe will yet prove to be among the richest in the world.
ARRIVAL OF THE EXPEDITION ON THE BANKS OF THE ZAMBESI.
Perhaps the portions of Africa which are the most interesting on account of geological investigations which have been made, are the valley of the Nile in Egypt, and the Desert of Sahara. It is well known that the river Nile annually overflows its banks in Egypt, and the inundation remaining a considerable period, a thin layer of soil is each year added to that which existed there before. This Nile mud, as it is called by geologists, has been the subject of considerable scientific examination for many years. In his work upon the “Geological Evidences of the Antiquity of Man,” Sir Charles Lyell gives a full account of certain systematic borings in the Nile mud which were made between the years 1851 and 1854, under the superintendency of Mr. Leonard Horner, but who employed to practically conduct the examinations an intelligent, enterprising, and faithful Armenian officer of engineers, Hekekyan Bey, who had for many years pursued scientific studies in England, was in every way qualified for the task, and, unlike Europeans, was able to endure the climate during the hot months, when the waters of the Nile flow within their banks. Sir Charles Lyell states that the results of chief importance arising out of this inquiry were obtained from two sets of shafts and borings—sunk at intervals in lines crossing the great valley from east to west. One of these consisted of fifty-one pits and artesian perforations made where the valley is sixteen miles wide between the Arabian and the Libyan deserts, in the latitude of Heliopolis, about eight miles above the apex of the delta. The other line of pits and borings, twenty seven in number, was in the parallel of Memphis where the valley is five miles wide. Besides Hekekyan Bey, several engineers and some sixty workmen, inured to the climate, were employed for several years, during the dry season, in the furtherance of these interesting investigations.
It was found that in all the works the sediment passed through was similar in composition to the ordinary Nile mud of the present day, except near the margin of the valley, where thin layers of quartzose sand, such as is sometimes blown from the adjacent desert by violent winds, were observed to alternate with the loam. A remarkable absence of lamination and stratification, the geologist goes on to say, was observed almost universally in the sediment brought up from all points except where the sandy layers above alluded to occurred, the mud closely agreeing in character with the ancient loam of the Rhine. Mr. Horner attributes this want of all indication of successive deposition to the extreme thinness of the film of matter which is thrown down annually on the great alluvial plain during the season of inundation. The tenuity of this layer must indeed be extreme, if the French engineers are tolerably correct in their estimate of the amount of sediment formed in a century, which they suppose not to exceed on the average five inches. It is stated, in other words, that the increase is not more than the twentieth part of an inch each year, or one foot in the period of 240 years. All the remains of organic bodies found during these investigations under Hekekyan Bey belonged to living species. Bones of the ox, hog, dog, dromedary, and ass were not uncommon, but no vestiges of extinct mammalia were found, and no marine shells were anywhere detected. These excavations were on a large scale, in some instances for the first sixteen or twenty-four feet. In these pits, jars, vases, and a small human figure in burnt clay, a copper knife, and other entire articles were dug up; but when water soaking through from the Nile was reached, the boring instrument used was too small to allow of more than fragments of works of art being brought up. Pieces of burnt brick and pottery were constantly being extracted, and from all depths, even where they sank sixty feet below the surface toward the central parts of the valley. In none of these cases did they get to the bottom of the alluvial soil. If it be assumed that the sediment of the valley has increased at the rate of six inches a century, bricks at the depth of sixty feet have been buried 12,000 years. If the increase has been five inches a century, they have lain there during a period of 14,400 years. Lyell states further on that M. Rosiere, in the great French work on Egypt has estimated the rate of deposit of sediment in the delta at two inches and three lines in a century. A fragment of red brick has been excavated a short distance from the apex of the delta at a depth of seventy-two feet. At a rate of deposit of two and a-half inches a century, a work of art seventy-two feet deep must have been buried more than 30,000 years ago. Lyell frankly states, however, that if the boring was made where an arm of the river had been silted up at a time when the apex of the delta was somewhat further south, or more distant from the sea than now, the brick in question might be comparatively very modern. It is agreed by the best geologists that the age of the Nile mud cannot be accurately, but only approximately calculated by the data thus far furnished. The amount of matter thrown down by the waters in different parts of the plain varies so much that to strike an average with any approach to accuracy must be most difficult. The nearest approach, perhaps, as has been observed by Baldwin, to obtaining an accurate chronometric scale for ascertaining the age of the deposits of the Nile at a given point, was made near Memphis, at the statue of King Rameses. It is known that this statue was erected about the year 1260 B. C. In 1854 it had stood there 3,114 years. During that time the alluvium had collected to the depth of nine feet and four inches above its base, which was at the rate of about three and a half inches in each century. Mr. Horner found the alluvium, below the base of the statue, to be thirty feet deep, and pottery was found within four inches of the bottom of the alluvium. If the rate of accumulation previous to the building of the statue had been the same as subsequently, the formation of the alluvium began, at that point, about 11,660 years before the Christian era, and men lived there some 12,360 years ago, cultivating the then thin soil of the valley. But it would appear to be certain that the average deposit is so slight annually that many centuries more than those formerly quite universally received as the age of the world for the stage of mankind’s achievements must have passed since the work of man’s hands have been buried under these vast deposits of alluvium. Thus, geology insists, is the fact of man’s existence, long before the historic era, conclusively established.
