2.

and in strata, from This bed is eight feet

Bituminous Coal, very pure; structure slaty; three

feet thick.

3. Water lime, in thin beds reposing on the coal. Thickness, six feet.

4. A Chloritic rock; color deep, almost verdigris green. Four feet thick.

5. Lias, which we have already described.

6.

Calcareous tufa. It is porous, as if pierced in all directions by small worms. Six feet thick.

7. Hard sparry limestone, of a light dove color, tinged with brown. Thirty feet thick.

8. Sandstone, the lower part in strata of a few inches thick, and contains some fossil remains. 100 feet thick.

9. Argillaceous, loamy soil, rich, and covered with timber. Ten feet thick.

TERTIARY STRATA.

The Tertiary, or third formation, as the name indicates, was deposited after the secondary, and may be considered as proceeding from the disintegration of this and the primary series.

With respect to its relative antiquity, the tertiary is newer than chalk, and older than the Diluvial and Alluvial deposites. When these, therefore occur in the series the tertiary formations are between the chalk and the diluvium.

The Tertiary strata consists of beds of clay, sand, marl, pudding-stones, and the newer limestone deposites, such as are found in the Paris basın, and in the Isle of Wight. These formations often contain abundance of fossil shells and plants, together with the bones of fish and quadrupeds. The famous locality of fossil fish at Monte Bolca, in Italy, is in tertiary strata.

In North America this formation is very extensive, reaching without interruption along the sea coast from Long Island to Louisiana and extending in some parts several hundred miles inland. It consists of sand and clay often mixed with an abundance of sea shells. The valley of the Connecticut is in a considerable proportion of the same formation, consisting of sand and clay, though the shells are absent. The whole of Long Island, Martha's Vineyard, and Nantucket are also tertiary formations. In general, tertiary strata show no marks of disturbance, being deposited since the lower rocks were disrupted.

VOLCANIC AND BASALTIC ROCKS.

These owe their origin to volcanic fire, and have been either ejected from burning mountains, or forced up to the surface of the earth in a melted state by volcanic action. Some of these rocks occasionally cover all the formations hitherto described, and as volcanoes are still active, they

may and indeed do, cover the most recent deposites of sand and gravel.

This division is known under the name of unstratified rocks, which also includes granite. Many geologists suppose that granite also had an igneous origin; and this indeed appears to be the prevailing opinion of the ablest writers of the day.

Basaltic or Trap rocks, including also those formed of lava, cover the other formations, in a very irregular, and uncertain manner. In France, large districts of country are buried under ancient lava, and the northern parts of Great Britain abound with basaltic rocks.

The word trap, is said to come from the Swedish trappa, which signifies a stair, or step, because rocks of this kind often separate in such a manner as to form stairs.

The application of this term is far from being definite, some geologists meaning by it such unstratified rocks as basalt, greenstone, porphyry, and their associates; while others have confined it to such rocks as are chiefly composed of hornblende, whether stratified or not. The former application of this term is undoubtedly the most common and appropriate.

The most important volcanic rocks are Basalt, Greenstone, and Lava.

Basalt. The color of this rock is dark grayish black, or brownish gray. It is found in large shapeless masses, or in columnar prisms, with from three to nine faces. These columns are of all sizes, from a few inches to several feet in diameter, and sometimes four hundred feet in height. They are composed of joints, or blocks of the same angular shapes, resting one upon another. The texture of basalt is fine grained, or compact, and it often contains other minerals imbedded in it, such as felspar, quartz, mica, leucite, and oxide of iron. It also exhibits hollow cavities, or vesicles, apparently formed by bubbles of air during its fusion. The Giant's Causeway in the North of Ireland, is composed of basaltic columns.

Greenstone. This is a compact, hard, tenacious rock of a dark grayish color, with a greenish tinge. It is essentially composed of hornblende and felspar. This rock occurs in beds of greater or less extent, sometimes forming extensive ranges of mountains. In this country green

stone is a common rock. The range of mountains on the west side of the Connecticut, reaching from New-Haven to Northampton, is of this rock. In some places, their height is several hundred feet. These rocks, as will be seen in another place, are undoubtedly of volcanic origin, having been elevated to their present situation through fissures, by the force of subterranean fire.

Lava. This term comes from the Gothic, and signifies to run, in reference to the flowing of volcanic matter.

The products of volcanic mountains often present very different appearances, and hence have received several names, as volcanic slags, volcanic enamel, cellular lava, compact lava, pumice, &c. But in general terms, all the liquified products of volcanoes are called lava, and for the purposes of elementary geology, this definition is perhaps sufficient.

The colors of lava are most commonly yellowish, or greenish gray sometimes running into sulphur yellow, and grayish black. Some are compact, while others are full of small pores, and others are fibrous with a silky lustre; but all the different kinds run into each other, so that it is often difficult to make distinctions between them.

DILUVIUM.

Diluvia, or diluvial deposites, are generally supposed to have been formed during the general deluge. They consist of sand, pebbles, and blocks, or fragments of various kinds of rocks, not generally existing in the districts where these deposites are found at the present day, and hence they must have been transported from a distance. In many instances, the diluvial rocks appear to have been moved from great distances, their dimensions and situations at the same time indicating a water power of much greater force, than any which has been described, except the Noachian deluge, and it is therefore considered reasonable to attribute these effects to that cause.-See Deluge.

ALLUVION.

Alluvia, or Alluvial deposites, are such accumulations of sand, mud, and soil, together with fragments of wood, as are constantly forming at the present day, by the currents of rivers and brooks, or by the rain which falls on hills and mountains. These are formed by causes now constantly operating, and we have shown that considerable changes have been wrought on the earth by such

causes.

We have now given a short account of each formation, and species of rock which compose the great bulk of the earth. There are, however, several rocks described in more extended treatises on this subject which we have omitted, and which occasionally form considerable hills, or underlay certain districts of country. This deficiency, with respect to names, will be supplied by the following view of M. Boue's classification of rocks, corrected and extended by Dr. Ure, of Glasgow. This contains the names of all the known members of each class, and by it the student will be enabled to observe the synonymous terms, with the classification we have employed.