full-grown Serpula, therefore, which now adheres externally, could not have begun to grow till the Micraster had died, and the spines became detached.

b

Fig. 12.

a

Now the series of events here attested by a single fossil may be carried a step farther. Thus, for example, we often meet with a seaurchin (Ananchytes) in the chalk (see fig. 12), which has the lower valve of a Crania, a genus of Brachiopoda, fixed to it. The upper valve (b, fig. 12) is almost invariably wanting, though occasionally found in a perfect state of preservation in white chalk at some distance. In this case, we see clearly that the sea-urchin first lived from youth to age, then died and lost its spines, which were carried away. Then the young Crania adhered to the bared shell, grew and perished in its turn; after which the upper valve was separated from the lower before the Ananchytes became enveloped in chalky mud. The rate of accumulation of the chalk must therefore have been very slow.

b. Upper valve of Crania detached.

It may be well to mention one more illustration of the manner in which single fossils may sometimes throw light on a former state of things, both in the bed of the ocean and on some adjoining land. We meet with many fragments of wood bored by ship-worms, at various depths in the clay on which London is built. Entire branches and stems of trees, several feet in length, are sometimes found drilled all over by the holes of these borers, the tubes and shells of the mollusk still remaining in the cylindrical hollows. In fig. 14, e, a representation is given of a piece of recent wood pierced by the Teredo navalis, or common ship-worm, which destroys wooden piles and ships. When the cylindrical tube d has been extracted from the wood, the valves are seen at the larger or anterior extremity, as shown at c. In like manner, a piece of fossil wood (a, fig. 13) has been perforated by a kindred but distinct genus, the Teredina of Lamarck. The calcareous tube of this mollusk was united and as it were soldered on to the valves of the shell (b), which therefore cannot be detached from the tube, like the valves of the recent Teredo. The wood in this fossil specimen is now converted into a stony mass, a mixture of clay and lime; but it must once have been buoyant and floating in the sea, when the Teredina lived upon, and perforated it. Again, before the infant colony settled upon the drift wood, part of a tree must have been floated down to the sea by a river, uprooted, perhaps, by a flood, or torn off and cast into the waves by the wind; and

thus our thoughts are carried back to a prior period, when the tree grew for years on dry land, enjoying a fit soil and climate. Fig. 13.

Fossil and recent wood drilled by perforating Mollusca.

Fig. 13. a. Fossil wood from London clay, bored by Teredina, .

b. Shell and tube of Teredina personata, the right-hand figure the ventral, the left the dorsal view.

Fig. 14. e. Recent wood bored by Teredo, 1.

d. Shell and tube of Teredo navalis, from the same.

c. Anterior and posterior view of the valves of same detached from the tube, nat. size.

The present rate of accumulation of deep sea sediment is exceedingly slow, as is proved by the growths of coral that occur on electric cables. The corals grow at great depths very much quicker than the accumulation of the foraminiferal oaze. But rapid accumulation of some sediments must have taken place formerly, for tree stems standing erect are found in strata of coal, sand, and grit which gathered around them.

It has been already remarked that there are rocks in the interior of continents, at various depths in the earth, and at great heights above the sea, almost entirely made up of the remains of zoophytes and testacea. Such masses may be compared to modern oyster-beds and coral-reefs; and, like them, the rate of increase must have been extremely gradual. But there are a variety of stone deposits in the earth's crust, now proved to have been derived from plants and animals of which the organic origin was not suspected until of late years, even by naturalists. Great surprise was therefore created some years since by the discovery of Professor Ehrenberg, of Berlin, that a certain kind of 1 Duncan, Proc. Roy. Soc. vol. xxvi. p. 133, 1877.

siliceous stone, called tripoli, was entirely composed of millions of the remains of organic beings, which were formerly referred to microscopic Infusoria, but which are now admitted to be plants. They abound in rivulets, lakes, and ponds in England and other countries, and are termed Diatomaceæ by those naturalists who believe in their vegetable origin. The subject alluded to has long been well known in the arts, under the name of Infusorial Earth or Mountain Meal, and is used in the form of powder for polishing stones and metals. It has been procured, among other places, from the mud of a lake at Dolgelly, in North Wales, and from Bilin, in Bohemia, in which latter place a single stratum, extending over a wide area, is no less than 14 feet thick. This stone, when examined with a powerful microscope, is found to consist of the siliceous tests of the Diatomaceæ figured below, united together without any visible cement. It is difficult to convey an idea of their extreme minuteness; but Ehrenberg estimates that in the Bilin tripoli there are 41,000 millions of individuals of the Gallionella distans (see fig. 16) in

every cubic inch (which weighs about 220 grains), or about 187 millions in a single grain. At every stroke, therefore, that we make with this polishing powder, several millions, perhaps tens of millions, of perfect fossils are crushed to atoms.

A well-known substance, called bog-iron ore, often met with in peat-mosses, has been shown by Ehrenberg to consist of innumerable articulated threads, of a yellow ochre colour, composed of silica, argillaceous matter, and peroxide of iron. These threads are the cases of a minute microscopic body, called Gallionella ferruginea (fig. 15) associated with the siliceous plates of other freshwater algæ. Layers of this iron ore occurring in Scotch peat bogs are often called 'the pan,' and are sometimes of economical value.

It is clear that much time must have been required for the accumulation of strata to which countless generations of Diatomaces have contributed their remains; and these discoveries lead us naturally to suspect that other deposits, of which the materials have been supposed to be inorganic, may in reality be -composed chiefly of microscopic organic bodies. That this is

the case with the white chalk, has often been imagined, and is now proved to be the fact. It has, moreover, been lately discovered that the chambers into which these Foraminifera are divided are actually often filled with thousands of well-preserved organic bodies, which abound in every minute grain of chalk, and are especially apparent in the white coating of flints, often accompanied by innumerable needle-shaped spicule of sponges (see Chap. XVIII.)

The dust we tread upon was once alive!-BYRON.

How faint an idea does this exclamation of the poet convey of the real wonders of nature for here we discover proofs that the calcareous and siliceous dust of which whole hills are composed has not only been once alive, but almost every particle, albeit invisible to the naked eye, still retains the organic structure which, at periods of time incalculably remote, was impressed upon it by the powers of life.

Freshwater and marine fossils.—Strata, whether deposited in salt or fresh water, have the same forms; but the embedded fossils are very different in the two cases, because the aquatic animals which frequent lakes and rivers are distinct from those inhabiting the sea. In the northern part of the Isle of Wight formations of marl and limestone, more than 50 feet thick, occur, in which the shells are of extinct species. Yet we recognise their freshwater origin, because they are of the same genera as those now abounding in ponds, lakes, and rivers, either in our own country or in warmer latitudes.

In many parts of France, in Auvergne, for example, strata occur of limestone, marl, and sandstone hundreds of feet thick, which contain exclusively freshwater and land shells, together with the remains of terrestrial quadrupeds. The number of land shells scattered through some of these freshwater deposits is exceedingly great; and there are districts in Germany where the rocks scarcely contain any other fossils except snail-shells (helices); as, for instance, the limestone on the left bank of the Rhine, between Mayence and Worms, at Oppenheim, Findheim, Budenheim, and other places. In order to account for this phenomenon, the geologist has only to examine the small deltas of torrents which enter the Swiss lakes when the waters are low, such as the newly formed plain where the Kander enters the Lake of Thun. He there sees sand and mud strewn over with innumerable dead land shells, which have been brought down from the valleys in the Alps in the preceding spring, during the melting of the snows. Again, if we search the sands on the borders of the Rhine, in the lower part of its course, we find.

countless land shells mixed with others of species belonging to lakes, stagnant pools, and marshes. These individuals have been washed away from the alluvial plains of the great river and its tributaries, some from mountainous regions, others from the low country.

Although freshwater formations are often of great thickness, yet they are usually very limited in area when compared to marine deposits, just as lakes and estuaries are of small dimensions in comparison with seas.

The absence of many fossil forms usually met with in marine strata, affords a useful negative indication of the freshwater

Fig. 18.

Fig. 19.

Cyclas (Sphærium) corneus, Sow. ;
living and fossil, nat. size.

Cyrena (Corbicella) Aluminalis, Möll.; fossil, Grays, Essex, and living in the Nile, nat. size.

origin of a formation. For example, there are no sea-urchins, no corals, no chambered shells, such as the nautilus, nor microscopic Foraminifera in lacustrine or fluviatile deposits. In distinguishing the latter from formations accumulated in the sea, we are chiefly guided by the forms of the mollusca. In a freshwater deposit, the number of individual shells is often as great as in a marine stratum, if not greater; but there is a smaller variety of species and genera. This might be anticipated from

the fact that the genera and species of recent freshwater and land shells are few when contrasted with the marine.