Page:Popular Science Monthly Volume 13.djvu/264

252 a blue color wherever the perchloride is unaltered, all the rest of the surface remaining white. The image is then freely washed in water, and passed through a bath of chlorhydic acid (8 to 10 parts to 100 of water), which removes the protoxide of iron salt; it is then washed again in water, and finally dried. The drawing then appears in blue lines on the pure white ground of the paper.

The Chinese Loess or Loam Deposits.—The origin of the "loess" deposits of China has long been a perplexing problem for geologists. This deposit is spread almost continuously over an area as large as the German Empire, besides existing in detached areas of nearly half that extent. Usually, the loess is several hundred feet in thickness, and in some places as much as 1,500 or even 2,000 feet. It is an earthy substance, of a brownish-yellow color, friable, chiefly consisting of argillaceous materials, with a small proportion of carbonate of lime; it has also mixed with it more or less of fine sand, the grains of which are very angular. The Baron von Richthofen, in his work on "China," the first volume of which has appeared, offers the most satisfactory theory yet presented of the origin of this loess. A very clear statement, both of the problem itself and of Von Richthofen's solution of it, is given by Prof. J. D. Whitney, in the American Naturalist, who states that the first geologist to notice and describe these remarkable deposits was Prof. Pumpelly. According to him, the loess of China is a lacustrine formation, each of the basins in which it occurs having been once the bed of a lake. But the absence of stratification and of fresh-water shells, and the presence of the bones of land-animals, appear to be utterly incompatible with this theory. Besides, the loess indicates by its structure the growth on its surface of an abundant vegetation. But a greater difficulty still stands in the way of the theory of a lacustrine origin—namely, the fact that everywhere the loess plainly shows itself to be a deposit which was not laid down till after the surface of the country had assumed its present configuration. Hence Richthofen unhesitatingly declares himself in favor of a subaërial origin of the loess. Wind and rain are, according to him, the agencies which produced these deposits. In the first place, he assumes the district of the loess to have been once destitute of outward drainage, and to have, in fact, consisted of a number of closed basins, such as are still found in the adjacent region, to the west, in Central Asia. These closed basins were prairies, and the loess is the collective residue of innumerable generations of herbaceous plants. It is the inorganic residuum which has accumulated during an immense lapse of time, as the result of the decay of a vigorous prairie-growth, ever renewing itself on the surface of the slowly-accumulating deposit. But how is the increase of the deposit provided for by the theory? Unless there be some source supplying material from without, there can evidently be no gain in thickness, however many generations of plants succeed each other. The necessary addition of mineral matters Richthofen considers to have been brought into these basins by two agencies, the rain and the wind, and the latter especially plays an important part in his theory. Each basin being surrounded by a rim of rocks, constantly undergoing decomposition, the particles thus set free were either swept down the mountain-sides toward the central area by rain, or blown thither by air-currents, and, once entangled among the vegetation, could not be cariedcarried [sic] farther.

The Pennsylvania Oil-Regions.—The oil-regions of Pennsylvania are, in an article by M. C. A. Ashburner, in the Journal of the Franklin Institute, divided into three districts, the southwestern, the western, and the northern, the southwestern lying south of the Ohio and west of the Monongahela, the western occupying the water-basin of the Alleghany, between Pittsburg on the south and the Philadelphia & Erie Railroad on the north, and the northern district extending north from the line of the same railroad. In the first of these districts the petroleum comes from the highest rocks, and in the third from the lowest, while in the second it comes from the rocks intermediate between the two. The "oil-sand group" of the southwestern district is composed of three sandstone members, separated by intervals containing coal-seams,