been deposited from chemical solution; on the other hand, it bears marks of a mechanical deposit, as if from water loaded with it in fine division. And upon this principle some gleam of light may perhaps be thrown upon the enigmatical appearance of the flints; for it is found that if finely divided silica be mixed with other earthy bodies, and the whole diffused through water, the grains of silica have, under certain circumstances, a tendency to aggregate into small nodules; and in chalk some grains of silica are discoverable." The flints occur in nodular masses of irregular forms, varying from less than an inch to more than a yard in circumference; they are quite insulated from each other, each one being entirely enveloped by the chalk. As they almost invariably include some organic body, they appear to be silicious aggregations about nuclei, as septaria are aggregations of calcareous and argillaceous matter. It has before been shown (§ 66,) that silica is soluble in water, as it occurs in the Geysers of Iceland. The chalk resembles the calcareous mud that accumulates in the lagoons of coral islets, produced by the ocean wearing corals and shells to a fine powder. (§ 79.)

253. The igneous rocks associated with the chalk and other members of the Secondary series, are basalt and other forms of trap. The basalt of the Giant's Causeway breaks through and overlies the chalk in the north of Ireland, the heat of the igneous rock having rendered the chalk hard and crystalline like primary marble.

CHAPTER VIII.

ROCKS OF THE TERTIARY PERIOD.

254. THE Tertiary embraces the strata of sand, clay, and limestone, which lie between the chalk and the superficial deposits designated Drift and Alluvium, with which they were confounded previous to the labors of Cuvier and Brogniart in A. D. 1810. They differ from the Secondary you did aby rocks in mineral constitution and especially in their organic Lifest contents; but are not very distinctly bounded at their upper limit, frequently merging in the recent deposits. In genferal they are not so firmly aggregated nor so thick as the

Secondary rocks, and are remarkable for exhibiting frequent allenabem alternations of marine and fresh-water formations in the same localities. These beds very frequently occur in limited and detached basins, as if deposited in shallow lakes or estuaries; they are however often continuous over very extensive tracts of country, as from Martha's Vineyard southward along the whole extent of the Atlantic coast of the United States. The order of succession of strata is not

orders!censo uniform as in the older rocks; and hence arises the difficulty of framing a description that will apply to them in all localities. Their alternations have, however, been accurately ascertained, especially in the Paris and London basins.

255. The sands and clays predominate in the Tertiary series; their colors are various, depending principally upon

the colored compounds of iron, and they are not sufficiently indurated to form firm sandstones and shales. The calcare

ous strata are more varied in structure and appearance, con- ston sisting of soft marls filled with shells-of rough masses of coral-of fresh-water beds of hard limestone-and of marine limestone of coarse sandy texture. They also contain beds of silica, buhrstone, gypsum, and rock salt.

256. The Tertiary strata occupy a large portion of the surface of Europe, conforming in a remarkable degree to the present outline of the sea; so that if the continent were depressed a few hundred feet, the sea would cover them, indicating that no essential change has occurred in the figure of the land since that period. The Tertiary series has been recognized also, in Northern Africa, in Asia, in North America, and in South America on both sides of the Andes.

257. These rocks were, at first, classified in accordance with their alternations as marine and fresh-water deposits; but these alternations are found not to be uniform. The classification generally adopted is that of MM. Lyell and Deshayes, based upon the relative proportion of shells specifically identical with those occurring at the present time. Shells are selected as the test because they are more generally and uniformly diffused in strata than the remains of any other class of animals.

EOCENE, (Eos, the dawn; and kainos, recent,) the dawn of the

present period with its races of plants and animals;
contains less than five per cent. of living species, in
its fossil contents.

MEIOCENE, (Meion, less; kainos, recent,) though containing
more living species than the Eocene, they are still
less than half; its per centage of recent species is
eighteen.

PLEIOCENE, (Pleion, more; kainos, recent,) a majority of whose fossils belong to existing species.

PLEISTOCENE, (Pleistos, most; kainos, recent,) a still nearer approximation to the present period, having ninety-five

per cent. of recent shells.

THE EOCENE OR OLDER

TERTIARY.

258. The Eocene deposits are the most distinct of the series, and occur well developed in France and England. They, however, vary much in local character. The beds of the Paris and London basins are of the same age, but differ greatly in mineral constitution; in the London group those of a mechanical origin-sands and clays almost exclusively prevail; while beds of limestone, silica, and gypsum abound in the Paris basin.

The following tabular arrangement exhibits the order of the beds in the two basins :

F represents the London Clay; E the plastic clay; D the Chalk; o the Wealden; B the Oolite; the Bagshot sand caps and overlies the London Clay. The position of the City of London is indicated by the building.

259. There are numerous other localities of the Eocene Tertiary, as in Southern France, Spain, Belgium, Italy, Greece, Asia Minor, Egypt, India and North and South. America.

260. The fossils of this group are very numerous and important, indicating the condition of the earth at the time of their deposition. The general character of the flora and fauna was like that within the tropics of the present day. The species are with very few exceptions extinct, and seem to have been almost exclusively confined to this formation. More than a hundred species of plants have been distinguished, many of which are dicotyledonous, and resemble intertropical plants of the present day; a great number of their seeds and fruits have been found in the London clay beds of the Isle of Sheppey. Various fossil resins have been discovered in these beds, the most important of which is amber, occurring in nodules from the size of a nut to that of a man's head; one specimen weighs 18 lbs. Specimens frequently contain insects whose positions and detached legs and wings indicate that they struggled after