Page:Popular Science Monthly Volume 9.djvu/402

380 might have existed during a very long time, instead of for a few weeks, as happens after a modern winter. Again, the slackened or suspended flow of the water, caused by such ice-obstructions, would have favored the deposition and accumulation about them of drift, and some may have thus been converted into complete dams. This process might occasionally have wholly filled with earthy material a gorge or narrow valley—as in the Niagara River—so as to block up and divert the course of the stream.

In view of these probable conditions of the river-valleys during the glacial flood, the question arises whether the height of the upper terraces above the narrows, on the rivers of Connecticut, was not partly owing to the existence of ice-obstructions. That this was so seems highly probable; and the height of modern spring-floods in the Connecticut at Middletown and Hartford is now often due, in part, to this very cause. If such obstructions existed in the Thames, Connecticut, and Housatonic Valleys, they were only partial obstructions, for in the case of each the terrace of the valley below the narrows declines quite gradually in height from the level above the narrows, instead of abruptly. Moreover, the material of the terraces below the narrows is like that above. Further evidence of the existence of such ice-barriers is to be looked for in a distribution of gravel and large bowlders across the valley just above the narrows, where the ice-masses had been brought to a stop and piled up. Prof. Dana has as yet observed no satisfactory evidence of this kind, but thinks the question needs more investigation.

Where the Army-Worm Moth lays its Eggs.—The mode of oviposition in Leucania unipuncta (the army-worm moth) was, till the other day, an unsolved problem in entomology. During the current year Prof. C. V. Riley, State Entomologist of Missouri, undertook the methodical investigation of this subject, and at the meeting of the St. Louis Academy of Science, on May 1st, was able to announce that his researches had been entirely successful. Guided by the structure of the ovipositor, Mr. Riley concluded that the moth would naturally secrete the eggs where they could not be easily seen. This conclusion was afterward verified by direct observation, the author having witnessed the mode of oviposition on blue grass. The eggs are, as he surmised, secreted, being either glued in rows of from five to twenty in the groove which is formed by the folding of the terminal grass-blade, or else between the sheath and the stalk. The eggs are white, slightly iridescent, spherical, .02 inch in diameter. They are fastened to each other and to the leaf, and covered along the exposed portion by a white, glistening, viscid substance. As they mature the color becomes yellowish, and by the seventh day the brown head of the embryo shows distinctly through the shell. The larva emerges from the eighth to the tenth day, is 1.7 millimetre in length, dull, translucent white in color, with a large black-brown head, and is a looper, the two front pairs of abdominal prolegs being atrophied. On account of its extremely small size and of the color resembling the pale bases of the grass-stalks near the ground, it is almost impossible to find them even where they are numerous. The one great economical result of these researches is the indicating of an effectual mode of destroying the army-worm—viz., burning the eggs with the stubble.

How the Mississippi wears away its Banks.—The observation is made by Reclus, in his work "The Earth," that the Mississippi River seems to contradict the law of displacement of running water, which in consequence of the motion of the earth on its axis causes all streams which flow north or south to hug the west side of their valleys. But Mr. G. W. R. Bailey, in a paper published in the Journal of the American Society of Civil Engineers, shows that the anomaly is an apparent one only. "The river," he writes, "does wear away its western shoreline more rapidly than the eastern, but it cannot do otherwise than gradually excavate circular bends, of from twenty to twenty-five miles in circumference generally, and then cut them off, leaving them to the westward. There has been, always, an excess of overflow and of sedimentary deposits west, and by far the largest number, as well as the greatest bends when cut off, have been left to the west. The western portion