It is perfectly white, and is mainly composed of bleached shells, and its thickness is known to be more than twelve feet.

Carbonate of lime seems to be chemically deposited in some Irish lakes, in the form of a white, mealy substance, which is thus described by Mr. Jukes. "The beds of the lakes in the limestone district of Ireland have often a thick deposit of white mud, which, when dry, is almost like flour in appearance, and is wholly soluble in acids. It is full of undecomposed fresh-water shells of ordinary living species, but does not itself disclose any trace of an organic origin. My friend, Dr. J. Barker, of Dublin, subjected some of it, at my request, to a careful microscopical examination, but could discover no trace of organic structure. Around lakes that have been partially drained, large deposits of this substance, several feet in thickness, may be seen; and, in sounding some of the lakes, I usually found the lead sank into and came up partially coated with this substance. On some parts of the shores of the lakes there are accumulations of small nodular concretionary-looking balls of about half-an-inch in diameter, which Dr. Ållman, of Edinburgh, told me were a species of Nullipore."

2. Siliceous.-Deposits, consisting largely of silica, are formed in some lakes by the agency of minute vegetable organisms called Diatoms, which are able to secrete and appropriate the silica dissolved in the water, just as the molluscs secrete the carbonate of lime to form their shells. Diatoms consist of simple cells, either single, or united in a linear series, and each is enclosed in a siliceous case or shell; these cases are often elegant in shape and delicately sculptured on the surface. They are only visible under the microscope, but compensate for their minuteness by their extraordinary abundance, being so prolific that the number of individuals derived from a single diatom in one month would form a bed of silica, 25 square miles in extent and 14 foot thick (Ehrenberg).

Diatoms are found in the sea, as well as in bogs, stagnant pools and lakes, and thick deposits are sometimes formed by the accumulation of their siliceous cases, or

1 "Manual of Geology," third edition, p. 384.

frustules. Such is the berg-mehl (or mountain-meal) on the shores of certain lakes in Sweden. This is of such a fine, floury consistency that the inhabitants use it to mix with flour. Diatom earth is also used to make dynamite by mixing it with nitro-glycerine.

In some Nova Scotian lakes, according to Dr. Dawson,1 the layers of Diatom earth are several feet thick; he describes it as "a white and, when dry, very light, friable, earth, having a floury texture, and showing, when examined in a bright light, an infinity of minute, shining specks." The white clay of Lake Superior may perhaps be of a similar nature (see p. 173).

Fig. 38. Diatoms and Spicules of Spongillæ from the banks of the river Bann.

A bed of similar Diatom earth occurs near the base of the Mourne Mountains, county Down, Ireland. It is described by Dr. Drummond as forming a very light, white substance, resembling in appearance carbonate of magnesia. On the banks of the river Bann, in the same county, there is an extensive stratum of similar earth (see fig. 38). The siliceous frustules belong to species of Navicula, Gallionella, Coscinodiscus, Gomphonema, Bacillaria, &c., together with the spicules of fresh-water sponges (Spongille).

At Bilin, in Bohemia, there is an ancient deposit of siliceous material, 14 feet thick, which is entirely composed

of diatoms, and chiefly of a species called Gallionella distans. Ehrenberg calculated that a cubic inch of the earth contained about forty-one thousand millions of organisms. It is known locally as polierschiefer, being used by lapidaries for polishing purposes. Similar stone found in Italy and elsewhere is called Tripoli.

An enormous deposit of infusorial silica, 250 feet thick, has been discovered in the Kawsoh Mountains, Nevada, U.S. Mr. Clarence King' describes the rock as soft and earthy in texture, pure white in colour, and so light as to float on water. Some of the single beds are 8 to 10 feet thick, and about 90 per cent. of the mass is pure silica. The diatoms are referable to 46 different species, of which the most abundant are Gallionella granulata, G. sculpta and Spongolithis acicularis.

Fig. 39. Gallionellæ (highly magnified).

3. Ferruginous. The substance known as Bog-iron-ore was also shown by Ehrenberg to be the product of a particular Diatom called Gallionella ferruginea, which forms long threads or filaments consisting of a series of minute cells. Professor Bailey thus describes their appearance in the pools near West Point, United States:-"The bottoms of these are literally covered in the first warm days of spring with a ferruginous-coloured mucous matter about a quarter of an inch thick, which, on examination by the microscope, proves to be filled with millions of these exquisitely beautiful siliceous bodies. Every submerged stone, twig, and spear of grass is enveloped by them; and the waving plume-like appearance of a filamentous body, covered in this manner, is often extremely elegant." The

1 66 'Exploration of the 40th Parallel," vol. ii.

iron ore formed in such pools and bogs consists chiefly of silica and peroxide of iron; it occurs in layers under some Scotch bogs, where it is generally called "the pan," and the Diatoms composing this pan probably lived in the lake which afterwards became a peat-growing bog.

CHAPTER XI.

MARINE DEPOSITS.

§ 1. Mechanically formed.

N describing the operations of a river, we have followed the course of the detritus transported by its current, and have seen, that though this is temporarily deposited at certain points along the river-valley, yet most of it is eventually moved on again and triturated into smaller and smaller particles. Sediment is thus continually brought down from higher to lower levels, and never finds a permanent resting-place until it is carried into a lake or into Even delta-mud is liable to removal, and from its mouth the river is constantly discharging a cloud of sediment, which is carried away by the tidal currents and spread over the sea-bottom.

the sea.

We must also remember that besides this visible sediment there is much invisible material dissolved in the water, and that this latter also is poured into the sea.

Excepting, therefore, the small portion which is intercepted by lakes, the sea is the ultimate recipient of all materials eroded from the land. We shall first consider what becomes of the matter in mechanical suspension, and, secondly, account for the disposal of that in chemical solution.

Assortment of Material.-Let us commence with the case of a rocky coast where small rivers with rapid currents bring down detritus of all kinds and sizes from large stones to fine mud. The order in which these materials are deposited must especially be noticed; as the river-current enters the sea it will first deposit the pebbles, next the smaller stones and coarse sand, farther out fine sand, and last of all, silt and mud. This order of succession is always maintained except where interrupted by local currents and influences. The width and extent of these deposits will,