them; and then the different sides of the vallies are of course of different materials.

The cause that produced vallies has, in many places, carried off the strata that once covered masses of rock which we now find bare or insulated hills: this disrobing of the rock has been termed Denudation.

*It is well known that not only Argillite, but every member of the primary class has been occasionally found in contact with Granite.

LECTURE III.

Changes produced on the Earth's Surface, by the formation of Peat, &c.-of Coral Reefs Volcanoes Observations on their Structure, &c.-Vesuvius-Etna -Sabrina Island-Earthquakes-those of Lisbon and Calabria Volcanic Fire-its Intensity-Situationand Origin.

Independent of the changes that are now constantly going on by the partial agency of water, we have other causes acting more powerfully in altering the present configuration of the Earth's surface. Of these I shall briefly notice but threeviz. :

I. The formation of Peat, Bogs, and Mosses.
II. The formation of Coral Reefs-and

III. Internal Heat, as manifested by Volcanoes and Earthquakes.

The change produced by the formation of Peat is partial and trivial compared with the others. Mountain and Marsh Peat are formed in the places whence their name is derived, by the de

composition of plants. In the shallow parts of lakes are found numerous subaquatic plants, which in summer flower at the surface and then sink to the bottom. By the annual death of a portion of these a stratum of peat is formed at the bottom, where this process is constantly going on. The death of fresh water shell fish, and the deposits of earth and sand brought into the lake by streams, assist in raising the bottom of the lake, and thus prepare it for other plants; in like manner to thrive, blossom and die.

In other cases no distinct bed of peat is produced from the subaquatic plants-but a stratum is formed by the decay of land plants, which gradually extends into the lake and finally occupies its bed. The shells, which for centuries have been deposited at the bottom of the lake thus become covered by peat, and are eventually consolidated into marl beds—thus offering one of the most useful auxilliaries to Agriculture.

By occasional floods, sand, gravel, clay and earthy particles are thrown over these thin strata of peat, and destroy vegitation. By many and alternate similar causes, a series of beds of peat and of soil is produced, which eventually, exhausting the lake, becomes a fertile field, instead of forming a peat or moss meadow.

Different kinds of peat have their peculiar plants entering into their formation.

Mountain Peat is formed principally by the fol

lowing, viz. :

Erica cinerea,

vulgaris,
tetralix,

Myrica gale,

Empetrum nigrum,
Tormentilla erecta,

Arbutus uva ursi,

Vaccinium vitis idæa,

Juniperus communis,

Polytrichum commune,
Lycopodium clavatum,
Lichen rangiferinus,
Nardus strictus,

Scirpus cæspitosus,
Juncus squarrosus,

Many grasses of the genera,
Aira, agrostis, and carex.

In the formation of Marsh Peat, the sphagnum palustre is the chief ingredient, assisted materially

however by the following, viz. :

Bogs and Mosses are also accumulations of vegitable matter in wet ground-settling in successive generations on itself, until its bulk rises considerably above the level of its bed. The surface of a bog is always undulated, and terminates abruptly, sometimes almost perpendicularly. The average height of the great Irish bogs is about 250 feet above high water mark in Dublin harbour.

Quaking bogs are produced in wet, flat grounds, where springs abound. Weeds, shrubs, and trees, by their decay and fall, assist in daming up the stream, and the water becoming stagnant, the whole flat is overflowed. A coarse grass, which is peculiar to these bogs, springs up in tufts: the roots become closely interwoven, and in a few seasons they grow