Page:Popular Science Monthly Volume 11.djvu/525

Rh toward the hatching-grounds of their parents, so that two years seem to be required to complete the migration cycle. The methods of prevention proposed are the cultivation and preservation of forest-trees; the protection of the prairie-grass till the appropriate time for destroying the young insects by fire; and the protection of insect-catching birds.

A New Wheat-Fungus in California.—Early in March of the present year the young wheat in certain districts of California began to exhibit on its expanded leaves yellowish-white patches of fungous growth. This peculiar species of "rust" or "mildew" appears to be new to California, and has been recognized by Mr. H. W. Harkness, of the San Francisco Microscopical Society, as the Erysiphe graminis of De Candolle. According to Mr. Harkness, the fungus appears on the expanded leaves in felt-like patches, from one-sixteenth to one-half inch in length, following the longest diameter of the leaf equally on both sides. It adheres with great tenacity, though, on examining sections of the leaf and fungus, no suckers are apparent. The leaf, at a short distance from the culm, soon turns brown and dries. In the earlier stages the mycelium is observed creeping over the surface, its filaments overlying one another until it forms a felted mass. To what extent the wheat will be damaged, it is as yet impossible to say. Adhering, as it does, closely to the plant, it doubtless appropriates the juices needed for maturing the grain, at the same time excluding air and sunlight from the tissues. Its visible effect is a weakening of the stalk, thus favoring decay.

A New Measure of Geological Time.—The amount of solid matter abraded from the land and carried to the sea as sediment, or in suspension, has often been estimated by geologists, and the importance of this suspended matter as an agent of geological changes has been fully recognized. But less attention has been bestowed upon the soluble matter washed out of the earth by every fall of rain and added to the waters of the ocean. This subject has been studied by Mr. T. Mellard Reade, whose results, as I set forth in his presidential address to the Liverpool Geological Society, we find summarized in the Nineteenth Century as follows: The author's first problem was to estimate the total quantity of solid material removed in the course of one year, by the solvent action of rain, from the entire surface of England and Wales, supposing the mean rainfall to be thirty-two inches. It is worthy of note that the variation of rainfall in different parts does not affect the quantity of dissolved matter to anything like the extent that might have been anticipated. True, the hilly districts of the west, in Cumberland, Wales, Cornwall, and Devon, intercept a large quantity of rain; but, then, these collecting-grounds are composed of old rocks, ranging from Cambrian to Carboniferous; and such rocks are, to a great extent, insoluble. On the other hand, in the southern and eastern counties the rainfall is much less than in the west; but, then, the rocks belong generally to Secondary and Tertiary formations, and are tolerably soft and soluble. A kind of compensation is thus established, the total quantity of solid matter carried off in solution in a given time being much the same in one river as in another. Roughly speaking, it may be said that where the rainfall is greatest the solubility is least; where the rainfall is least the solubility is greatest. It is needless to follow the details of the calculation by which the author is finally led to the conclusion that about 8,370,630 tons of solids are annually removed in solution by the rivers of England and Wales. Distributing the denudation equally over the country, the total area being 58,300 square miles, we obtain a general lowering of the surface to the extent of .000077 of a foot in a single year; in other words, it would require 12,978 years to reduce the surface of England and Wales by one foot through the solvent action of rain alone. Fewer data exist for extending this interesting inquiry to the Continent of Europe, and fewer still when we pass to other parts of the world. But, making the best of available data, and proceeding on the principle that "Nature, on the whole, averages the results," Mr. Reade feels justified in assuming provisionally that about one hundred tons of rocky matter will be dissolved by