A few notices of geological formations furnished in the writings of travellers, have already been given in the geographical account of the provinces. The summary published by Davis is a well digested survey of the observations collected by the gentlemen attached to the embassies.[167]
LOESS-BEDS OF NORTHERN CHINA.
The loess-beds, covering a great portion of Northern China, are among the most peculiar natural phenomena and interesting fields for geological investigation on the world’s surface. Since attention was first directed to this deposit by Pumpelly, in 1864, its formation and extent have been more carefully examined by other geologists, whose hypotheses are now pretty generally discarded for that of Baron von Richthofen. The loess territory begins, at its eastern limit, with the foot hills of the great alluvial plane. From this rises a terrace of from 90 to 250 feet in height, consisting entirely of loess, and westward of it, in a nearly north and south line, stretches the Tai-hang shan, or dividing range between the alluvial land and the hill country of Shansí. An almost uninterrupted loess-covered country extends west of this line to the Koko-nor and head-waters of the Yellow River. On the north the formation can be traced from the vicinity of Kalgan, along the water-shed of the Mongolian steppes, and into the desert beyond the Ala shan. Toward the south its limits are less sharply defined; though covering all the country of the Wei basin (in Shensí), none is found in Sz’chuen, due south of this valley, but it appears in parts of Honan and Eastern Shantung. Excepting occasional spurs and isolated spots—as at Nanking and the Lakes Poyang and Tungting—loess may be considered as ending everywhere on the north side of the Yangtsz’ valley, and, roughly speaking, to cover the parallelogram between longs. 99° and 115°, and lats. 33° and 41°. The district within China Proper represents a territory half as large again as that of the German Empire, while outside of the Provinces there is reason to believe that loess spreads far toward the east and north. In the Wu-tai shan (Shansí), Richthofen observed this deposit to a height of 7,200 feet above the sea, and supposes that it may occur at higher levels.
The term loess, now generally accepted, has been used to designate a tertiary deposit appearing in the Rhine valley and several isolated sections of Europe; its formation has heretofore been ascribed to glaciers, but its enormous extent and thickness in China demand some other origin. The substance is a brownish colored earth, extremely porous, and when dry easily powdered between the fingers, when it becomes an impalpable dust that may be rubbed into the pores of the skin. Its particles are somewhat angular in shape, the lumps varying from the size of a peanut to a foot in length, whose appearance warrants the peculiarly appropriate Chinese name meaning ‘ginger stones.’ After washing, the stuff is readily disintegrated, and spread far and wide by rivers during their freshets; Kingsmill[168] states that a number of specimens which crumbled in the moist air of a Shanghai summer, rearranged themselves afterward in the bottom of a drawer in which they had been placed. Every atom of loess is perforated by small tubes, usually very minute, circulating after the manner of root-fibres, and lined with a thin coating of carbonate of lime. The direction of these little canals being always from above downward, cleavage in the loess mass, irrespective of its size, is invariably vertical, while from the same cause surface water never collects in the form of rain puddles or lakes, but sinks at once to the local water level.
One of the most striking, as well as important phenomena of this formation is the perpendicular splitting of its mass into sudden and multitudinous clefts that cut up the country in every direction, and render observation, as well as travel, often exceedingly difficult. The cliffs, caused by erosion, vary from cracks measured by inches to cañons half a mile wide and hundreds of feet deep; they branch out in every direction, ramifying through the country after the manner of tree-roots in the soil—from each root a rootlet, and from these other small fibres—until the system of passages develops into a labyrinth of far-reaching and intermingling lanes. Were the loess throughout of the uniform structure seen in single clefts, such a region would indeed be absolutely impassable, the vertical banks becoming precipices of often more than a thousand feet. The fact, however, that loess exhibits all over a terrace formation, renders its surface not only habitable, but highly convenient for agricultural purposes; it has given rise, moreover, to the theory advanced by Kingsmill and some others, of its stratification, and from this a proof of its origin as a marine deposit. Richthofen argues that these apparent layers of loess are due to external conditions, as of rocks and débris sliding from surrounding hillsides upon the loess as it sifted into the basin or valley, thus interrupting the homogeneity of the gradually rising deposit. In the sides of gorges near the mountains are seen layers of coarse débris which, in going toward the valley, become finer, while the layers themselves are thinner and separated by an increasing vertical distance; along these rubble beds are numerous calcareous concretions which stand upright. These are then the terrace-forming layers which, by their resistance to the action of water, cause the broken chasms and step-like contour of the loess regions. Each bank does indeed cleave vertically, sometimes—since the erosion works from below—leaving an overhanging bank; but meeting with this horizontal layer of marl stones, the abrasion is interrupted, and a ledge is made. Falling clods upon such spaces are gradually spread over their surfaces by natural action, converting them into rich fields. When seen from a height in good seasons, these systems of terraces present an endless succession of green fields and growing crops; viewed from the deep cut of a road below, the traveller sees nothing but yellow walls of loam and dusty tiers of loess ridges. As may be readily imagined, a country of this nature exhibits many landscapes of unrivalled picturesqueness, especially when lofty crags, which some variation in the water-course has left as giant guardsmen in fertile river valleys, stand out in bold relief against the green background of neighboring hills and a fruitful alluvial bottom, or when an opening of some ascending pass allows the eye to range over leagues of sharp-cut ridges and teaming crops, the work of the careful cultivator.
UTILITY AND FRUITFULNESS OF THE LOESS.
The extreme ease with which loess is cut away tends at times to seriously embarrass traffic. Dust made by the cart-wheels on a highway is taken up by strong winds during the dry season and blown over the surrounding lands, much after the manner in which it was originally deposited here. This action continued over centuries, and assisted by occasional deluges of rain, which find a ready channel in the road-bed, has hollowed the country routes into depressions of often 50 or 100 feet, where the passenger may ride for miles without obtaining a glimpse of the surrounding scenery. Lieutenant Kreitner, of the Széchenyí exploring expedition, illustrates,[169] in a personal experience in Shansí, the difficulty and danger of leaving these deep cuts; after scrambling for miles along the broken loess above the road, he only regained it when a further passage was cut off by a precipice on the one side, while a jump of some 30 feet into the beaten track below awaited him on the other. Difficult as may be such a territory for roads and the purposes of trade, the advantages to a farmer are manifold. Wherever this deposit extends, there the husbandman has an assured harvest, two and even three times in a year. It is easily worked, exceedingly fertile, and submits to constant tillage, with no other manure than a sprinkling of its own loam dug from the nearest bank. But loess performs still another service to its inhabitants. Caves made at the base of its straight clefts afford homes to millions of people in the northern provinces. Choosing an escarpment where the consistency of the earth is greatest, the natives cut for themselves rooms and houses, whose partition walls, cement, bed and furniture are made from the same loess. Whole villages cluster together in a series of adjoining or superimposed chambers, some of which pierce the soil to a depth of often more than 200 feet. In more costly dwellings the terrace or succession of terraces thus perforated are faced with brick, as well as the arching of rooms within. The advantages of such habitations consist as well in imperviousness to changes of temperature without, as in their durability when constructed in properly selected places, many loess dwellings outlasting six or seven generations. The capabilities of defence in a country such as this, where an invading army must inevitably become lost in the tangle of interlacing ways, and where the defenders may always remain concealed, is very suggestive.
Facade of Dwelling in Loess Cliffs, Ling-shí hien. (From Richthofen.)