177. When sandstone and salt rocks predominate in this forma tion, they produce barren soils. When they are united, as in Arabia and Africa, they check all vegetation, and reduce the country to a desert.

178. Secondary rocks contain few metallic ores; but they are the only repositories of beds of coal, gypsum, and rock-salt, and contain extensive beds of ironstone-all of which are among the most useful treasures of the mineral kingdom. Salt and ironstone sometimes occur in such masses as to form mountains.

At Cardona, near Montserrat, in Spain, is a valley surrounded by cliffs, and traversed by hills of solid salt 600 feet high, which [22] glitter in the sun like mountains of gems. Other mountains of salt are found in India.

179. The springs of secondary countries are seldom pure, almost always containing lime, especially in limestone countries. and usually some of the species of salts. In secondary countries, also, we find mineral springs most abundant; and it is only in these that salt springs are found.

180. ALLUVIAL STRATA are chiefly composed of gravel, sand, elay, loam, (which is a mixture of sand and clay,) and mould, which contains decayed vegetables, and forms the richest soil. They appear to be produced by the destruction of the rock formations.

181. Sometimes the strata of rocks are broken down by storms, earthquakes, or other violent causes. Whole mountains have

been known to fall; and sometimes lakes or rivers swell and burst their boundaries, and change the face of the country.

In 1797, the Peak of Sicalpa, in New-Grenada, fell during an earthquake, and overwhelmed the city of Riobamba. Several instances of both facts have occurred in Switzerland, France, and other countries.

182. But besides these sudden changes, the high and rocky portions of the earth are gradually worn away by the operation of the weather. Peaks and abrupt precipices are reduced to rounded summits and gentle declivities, and the most rugged regions become habitable.

183. The materials of the mountains are carried by the rivers and torrents to the lower portions of the country, and are reduced to the rounded pebbles and fine sand or soil of alluvions.

184. With these materials, valleys are filled up, and uneven surfaces are levelled; bars and islands are often formed at the mouths of rivers; and large quantities are carried to the beds of the ocean, which are sometimes thrown up again on its shores.

185. It is evident that extensive portions of the earth have been formed in this manner, from the fact that the remains of shells and animals, and even whole forests, are found buried at a great depth below the surface. Such instances occur on the Atlantic coast of the United States, in England, Ireland, and many other coun tries.

Indeed, new alluvions are frequently formed under our eyes; particularly on the banks of rivers.

Thus, in the Connecticut River, and many others, it is very common to see a portion of land washed away from one side of the stream, while an equal portion is thrown up on the other.

186. Besides these partial and limited alluvions, the whole surface of the earth is covered with strata of gravel, sand, and clay, of a uniform appearance, which rise far above the present level of the waters. These could not have been formed by any cause less extensive than a universal deluge, like that recorded in the Scriptures.

Hence these have been called diluvial strata. They are found containing the remains of animals now extinct, at the height of 16,000 feet on the Himmaleb [23] Mountains, and 7,000 on the Andes. They also contain fragments of rocks found only in distant countries, which nothing but such a violent convulsion could have transported. The diluvial strata of England enclose such as are not found in any country nearer than Norway; and immense blocks of Finland granite are found on the plains of Russia.*

187. The fragments and pebbles of alluvions are sometimes hardened into loose, friable rocks; and the beds of shells they con

* Buckland,

tain are concreted into a similar rock, called shell limestone, which abounds in the southern United States.

These, and some other portions of the alluvial strata, with the secondary rocks which lie next to them, have been lately arranged in a new class, called the tertiary formation.

188. The surface of alluvial countries is usually level, often entirely flat, and little elevated above the sea. They are also free from rocks; and when the soil is fertile, are easily cultivated.

189. The rivers run smoothly over deep beds, and are navigable almost to their sources. But the mouths of the rivers and the harbours of an alluvial coast, are usually obstructed with bars and shoals, which render them difficult of access, and liable to be filled up by new alluvions.

190. Alluvial coasts ascend gradually from the ocean, sometimes rising in little hills from the water, as on the southern shore of the Baltic. In other cases, they form marshy flats, or sandy plains called downs, which are liable to be overflowed by an irruption of the sea-as on the coasts of Holland, and the alluvial coast of North America. The waters on such coasts are shallow to a great distance from land; and the navigation generally unsafe.

191. The fertility of an alluvion depends on the nature of the original rocks from which it is formed, and the manner in which it is deposited.

192. The alluvions of the hard primitive rocks are generally composed of sand or gravel, and are therefore barren and unproductive. The sea and great lakes throw up the same materials on their shores; and rapid rivers leave no other but this sterile deposit, the current conveying the finer particles to the ocean.

193. The barren regions of Siberia, and the deserts of Arabia, are examples of sandy alluvions. The waters are usually brackish in an extensive, dry alluvion; but often very pure in other cases. The air is generally clear, dry, and healthy.

194. The alluvions formed from transition and secondary rocks, are generally rich, for reasons before stated; and those of limestone rocks especially so.

195. Basaltic and volcanic rocks also form rich alluvions. Even the rugged, glassy lavas decay in the course of years, as is seen in the neighbourhood of Mount Etna and Vesuvius; and form a soil of extraordinary fertility, rendered still more productive by the heat of the volcanoes.

196. River alluvions, formed from streams which flow with a gentle current, or overflow their banks at certain seasons, generally consist of the finer particles of rocks, mingled with the de- [24] cayed remains of leaves and vegetables, and form a very rich soil.

The vales of such streams are among the most fertile and beautiful regions on the globe.

The banks of the Mississippi and Connecticut rivers, of the United States, and the Rhine, the Nile, and the Ganges, are fine examples. Not a pebble is found on the Ganges for 400 miles from its mouth.

197. The narrow valleys of mountainous and rugged countries are peculiarly rich also; because they receive the fine particles of soil washed from the surrounding declivities.

198. Rich alluvions abound in the decayed remains of animals and vegetables, and in other substances easily dissolved by water. Hence they are peculiarly adapted to the nourishment of plants; but for the same reason, the springs are generally impure, and. the exhalations often render the air very unhealthy.

199. They frequently form extensive marshes or swamps, as in the southern United States, which increase both these evils. The alluvial tracts near Rome are infested with exhalations termed the malaria, which render them almost uninhabitable.

200. The sand of alluvial strata, and the clay, in the various forms of potter's clay, fuller's earth, ochre, &c. are mineral substances of the first importance for building, pottery, glass-making, and other arts.

201. Rich alluvions contain extensive beds of bog-iron ore; also beds of marl, or calcareous clay, used for manure; and of the peat or turf, composed of the fibres of vegetables, which forms an important fuel in Holland, and other alluvial tracts destitute of forests.

202. Sandy alluvions are usually less productive in minerals; but in Asia, Africa, and South America, gold and precious stones are frequently mingled with them, which render them valuable mines.

203. The same causes which give rise to alluvions, render them peculiarly liable to change.

Those which are formed at the mouths of rivers, in many cases, are continually extending into the sea by the accumulation of soil; as in the Nile, and the Mississippi.

The land at the mouth of the Mississippi is known to have extended; and is calculated to have advanced 15 miles during the last century. On the shores of France and Holland, places which were formerly harbours, are now at some distance inland. The coast at the mouth of the River Po, has advanced 42,000 feet since the year 1604, or 180 feet in a year,*

204. Alluvial sands are moved, and sometimes greatly extended, by the ocean and winds, particularly on low flat coasts; as at

* Cuvier.

Cape Henry in Virginia, and in Ireland. In this way also, the deserts of Persia and Africa are frequently enlarged.

At Cape Henry, trees are sometimes buried by the moving sands. In some parts of Ireland, houses and villages have been covered or surrounded by a desert of sand, during the last century; and the roofs still rise above the waste, in evidence of the change. A number of French villages on the Bay of Biscay [25] have been overwhelmed in a similar manner. The sands of the African deserts have been carried by the winds into Egypt, and have overwhelmed many fertile regions and celebrated cities of antiquity.

This desolation can only be arrested by watering and cultiva ting the soil, that it may produce vegetation to protect it from the winds. A fertile region is often reduced to a desert, like the site of the ancient Babylon, when invasion or oppression leads a nation to neglect agriculture.

205. In some parts of the earth are found plains of several hundred miles in extent, which are almost always alluvial; in Russia and Siberia, they are called steppes..

206. Those which are moist, or well watered, sometimes produce shrubs and trees. But often, the only vegetation is a coarse species of grass, which grows to the height of a man, and furnishes pasturage for vast herds of animals.

207. In North America, these are called prairies, or savannahs. They are found in every part of the United States between the Allegany and the Rocky Mountains; some of small size, but others extending as far as the eye can reach,