The phenomena presented by coast-dunes are repeated by the sand-hills of the desert. They are steepest on the side which faces the wind, and the sand often blows into curious craterlike forms, somewhat resembling volcanic cones, one side of which has been blown away. The crater-like shaped hills would appear to be more perfectly formed in inland deserts than on coast-dunes.

Although the surface of sand-hills and sand-deserts is so bare and arid, water may frequently be found at no great depth. In coast-dunes the sand is generally quite moist just below the surface. Where the shape of the dunes is constantly changing, fresh sand being blown up from the shore, the water is often brackish, but in fixed dunes the water is generally quite fresh. If the sand cover any wide area, and especially if it be underlain by an impervious bed, such water may often be got in fair quantity. All this may be easily explained by the rain falling on the sand and being here retained, partly between the grains of sand, and partly held up by the underlying clay.

But it is not so easy to understand how large quantities of water are stored beneath the sands of the Sahara, nor the source from whence the water is derived. The Moors have for long ages been aware of the existence of this store of water, and they have dug wells through the sand to reach it. The water occurs at various depths, but it generally rises to the surface when the water-bearing bed is reached. The French engineers have made a great number of bore-holes in various parts of the desert. The quantity of water thus obtained is often very great. One bore-hole yielded nearly 1,500,000 gallons per day; the water of this was employed to turn a water-mill. Small fish are sometimes thrown up with the water of these wells and bore-holes. The same fish occur in surface pools of water; it seems therefore probable that there may be some communication between the pools and the waterbearing stratum.*

Partly from a want of vegetation, and partly from its greater dryness, the sand of inland deserts is more easily moved by the wind than is the sand of the coast. In the deserts of Peru the crescent-shaped hillocks of sand are called Medanos. They are "from ten to twenty feet high, and have an acute crest. The inner side is perpendicular, and the outer or bow side forms an angle with a steep inclination downwards. When driven by violent winds the medanos pass rapidly over the plains. The

* Professor Ramsay suggests this in a note to his translation of Professor Desor's Memoir on the Sahara, "Geological Magazine," vol. i. p. 27. For further details on this most interesting region, see Rev. H. B. Tristram's "Great Sahara,” 1860.

smaller and lighter ones move quickly forward, before the larger; but the latter soon overtake and crush them, whilst they are themselves shivered by the collision. These medanos assume all sorts of extraordinary figures, and sometimes move along the plain in rows forming most intricate labyrinths.".

Even more remarkable than the medanos are those tall revolving columns of sand which are sometimes seen in the deserts of Africa and Australia. The sand is taken up by the whirling wind, and is carried along like a waterspout. Several such columns are often seen at the same time; they travel at a great speed, the lower ends touching the ground. Sometimes obscuring the sun, sometimes glowing in its rays, they seem to be veritable pillars of cloud and fire. It was a sand-storm with columns of sand such as these which played terrible havoc with the army of Cambyses in the Libyan desert.

Baddeley, in describing the dust whirlwinds of northern India, says: "A broad wall of dust is observed advancing rapidly, apparently composed of a number of large vertical columns, each preserving its respective position in the moving mass, and each column having a whirling motion of its own."

The continuous action of blowing sand wears the rocks in a peculiar manner. Mr. Bauerman thus describes the sands of part of Arabia Petræa:-" Sand-scored stones are abundant everywhere. . . . . As a rule, the hardest rocks are the best polished this being more especially the case with quartz, jasper, carnelian, and similar siliceous substances; while the limestones, in addition to being polished, are furrowed and scored in every direction, and their surfaces studded with numberless small reticulating grooves, resembling the hill-shading on a topographical map.

A similar polishing action is apparent on the harder rocks of our own coasts, where these rocks are exposed to the wear of sand. Professor Ramsay has suggested that the mushroomshaped and undercut rocks, which are so characteristic of many sandstone areas, may have been worn into their present shape by the action of wind, blowing the loose sand against the under surfaces of the rocks. No one who has visited such rocks during high winds will doubt that the wind and sand are capable of so cutting out rocks.

The erosive action of sand, when let fall in a continuous stream, or propelled by means of a blast, has been turned to account during the last few years in etching or deeply graving glass and metal; the effect is singularly sharp and beautiful.

Tschudi's "Travels in Peru," ch. ix. (quoted by Marsh, p. 602). The inner side of the hillock can only be "perpendicular" when the medanos are in motion.

All who have passed much time on sandy shores, or among sand-dunes, must have remarked the peculiar whistling sound produced by the friction of the particles of sand against each other. Similar sounds, but far louder, are produced when dry sand runs down a slope. Captain H. S. Palmer has described such sounds as heard at Jebul Nágús, near Mount Sinai. Coarse sand, derived from the waste of the sandstone hills, is carried by high winds up the slope of the hill; here it lies at the angle of rest (30°), and the slightest cause is sufficient to set it in motion. Sometimes the sound is as soft and low as the hum of a humming-top; sometimes it almost approaches the roar of thunder.

143

THE COLORADO POTATO-BEETLE.

BY W. S. DALLAS, F.L.S.

[PLATE CXXI.]

ancient times men noted especially the injuries done to their property by their larger and more powerful enemies. It was the boar that came out of the wood to lay waste the vineyard, and the wild beasts of the field that ruined the hopes of the husbandman. At the present day, in all civilised communities, the number of such destroyers is greatly limited; but on the other hand we are compelled to recognise a multitude of minute enemies, which make up for their smallness by their great abundance, and perhaps are all the more mischievous by reason of their individual insignificance. Among the foes of the agriculturist which have come into notice of late years, the insect which has been called the "Colorado Potato-bug," has not only attracted a good deal of attention in America, where it has inflicted serious injury on the potato crops, but has also raised considerable apprehensions on this side of the Atlantic; circumstances which may justify us in giving some account of its appearance and natural history.

This beetle was discovered by Messrs. Say and Nuttall during an early American exploring expedition in what was then known as the "Far West," on the banks of the Upper Missouri, towards the foot of the Rocky Mountains. It was described by Say in the third volume of the "Proceedings" of the Academy of Natural Sciences of Philadelphia, published in 1824, under the name of Doryphora 10-lineata. For many years afterwards nothing was known of it except that such a beetle did exist, its true home being among the Rocky Mountains, where it feeds upon a wild solanaceous plant (Solanum rostratum, Dunal) peculiar to that region. But during all this time the advance of a civilised population was going on with astonishing rapidity in the direction of the Rocky Mountains, converting the vast region west of the Mississippi-which in 1824 was still a wilderness inhabited only by Indians and hunters--into a more or less

settled and cultivated tract, and the settlement of the territory of Nebraska carried cultivation, and with it the potato, into the district inhabited by the Doryphora. The insect was not long in taking advantage of the abundant supply of suitable food thus offered to it. In fact, we may with some justice assume that it found in the cultivated potato a nourishment better adapted to its wants than that furnished by the native plant on which it had previously fed; for it seems to have set out almost immediately in the direction of the more highly cultivated districts, and spread eastward with great rapidity.

In the year 1859 it was still far west, being then at a distance of 100 miles west of Onaha city, in Nebraska; but within two years (in 1861) it reached the state of Iowa, over which it spread completely in about three years, and in 1864 and 1865 did great mischief to the crops. During these years the beetles were also very destructive in the state of Missouri, and in 1864 and 1865 they crossed the Mississippi and invaded Illinois in great force, causing much injury to the potatoes in the northwestern part of that State. A branch migration northwards commenced in 1862, when the beetle made a settlement in the south-west corner of Wisconsin; by 1866 it had spread over the whole State. During the next two years it completed its occupation of Illinois, and in 1867 passed thence into Indiana and the south-west angle of Michigan, where it was very abundant in 1868. In this year its presence was noted in Pennsylvania, but it was not until 1871 that the Quaker State was fairly invaded by the western beetle. In this year the beetles swarmed about Detroit, at the south-eastern angle of Wisconsin, and great numbers of them are said to have been carried down with floating rubbish and on board ship into Lake Erie, to be wafted along that sheet of water and landed on the Canadian shores, and on the shores of New York and Pennsylvania at the opposite end of the lake. In 1871 also it was reported as doing mischief to the potatoes in Ohio; and in 1873 it had crossed Pennsylvania and reached the district of Columbia, near Washington, and almost to the shores of the Atlantic near Baltimore. In the meantime the northern migration had carried the pest through Wisconsin into Minnesota and Dakota; and through Michigan into Canada, where it made its appearance in 1870. Its transportation into Canada was in part effected by means of the shipping on the lakes. In the south also Kentucky and western Virginia were invaded in 1871 and 1873.

In the year 1868, when the western potato-beetle had reached the centre of Indiana, Mr. B. D. Walsh estimated the rate of its advance at about 60 miles a year, and upon this foundation predicted that it would reach the Atlantic coast