Open main menu

Page:Popular Science Monthly Volume 24.djvu/257

This page has been validated.

some others, of its stratification, and from this a proof of its origin as a marine deposit.

But, since attention was first directed to this formation by Mr. Pumpelly, in 1864, its structure has been more carefully examined by other geologists, whose hypotheses are pretty generally discarded for that of Baron von Richthofen. This gentleman, who may be considered facile princeps among foreign geologists who have visited China, argues that these apparent layers of loess are due to external conditions, as of rocks and débris sliding from surrounding hill-sides upon the loess-dust 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-bottom, 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 marlstones, 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 some stream or road-bed, the traveler 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 unrivaled picturesqueness, especially when lofty crags, which some variation in the watercourse has left as giant guardsmen of 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 teeming crops, the work of the careful cultivator.

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 fifty or a hundred feet, where the passenger may ride for miles without obtaining a glimpse of field or landscape. Lieutenant Kreitner, of the Szechenyi exploring expedition (whose pleasant article on Thibet appeared in "The Popular Science Monthly" for August, 1882) illustrates, by a personal experience when in Shansi, the difficulty and dan-