pansive forces from below crack the solid earth above, obtain vent, and leave behind them vast fissures. Or fractures may arise from the sudden uplifting of an enormous bulk of solid rock, which, penetrating the crust, rises upward into the stature of a vast mountain, or perhaps even into a widely-extending range of mountains.

34. Overlying Strata.-Or the mass, so driven up and vomited forth, may be in a state of fusion, and, accordingly, spread itself over the neighbouring surface strata, and hence be called overlying.

35. False, or Interstratification.-Or, raised upwards less powerfully, the intruding granite may simply force its way between the superincumbent strata, and there remain as an example of false, or interstratification.

36. Dykes. Or, still more weakly impelled, it may find its way into some small existing fissure, and so form a dyke; and which does not always run merely in a vertical or inclined position towards the surface, but sometimes extends horizontally. Thus we sometimes see dykes of granite extending along the ground like an artificially raised wall, the softer strata in which they were imbedded having wasted away.

37. Veins. Or, lastly, the expanding force may drive the igneous-and in this case, evidently fused rock-into a great number of branching and minute crevices, to which geologists give the name of veins.

38. Faults. By the word fault is meant that some portions of rocks or strata, having been lifted and disruptured, the edges, in falling, have fallen into positions different from those they originally occupied as regards each other, even though again touching. Fig. 3 affords an example.

39. Slips.-A slip is a minor fault, and expresses the fact, that while an upheaved and divided body falls back again, each part with the same horizontal position as it before occupied, one side falls lower than the other. THE FORMS OF STRATA may be all comprised within the following:

This diagram is intended to show how different strata are found lying horizontally upon each other, and must not be supposed to represent any particular rocks.

41. (Fig. 3.) Inclined strata, with fault in the centre, filled with rubbish. Strata in this position must have been raised and dislocated by expansive

Fig. 3.

d

Vertical Strata and

Fault.

a

forces from beneath. The angle which inclined strata present to the horizon is called the dip, or angle of inclination.

30

42. (Fig. 4.) Vertical Strata.-This engraving presents a good example of a very interesting kind of stratification. "Vertical strata afford,' says Sir Charles Lyell, from whom we borrow this and one or two other illustrations, "the most unequivocal evidences of a change in the original position. We find in Scotland, in the northern skirts of the Grampians, beds of pudding stone, alternating with the Example of Vertical Conglolayers of fine sand, all placed vertically to the horizon. When Saussure first observed certain conglomerates in a similar position in the Swiss Alps, he remarked that the pebbles, being for the most part of an oval shape, had their longer axes parallel to the planes of their stratification. (See Fig. 4.) From this he inferred that such strata must, at first, have been horizontal, such oval pebble having originally settled at the bottom of the water, with its flatter side parallel to the longer, for the same reason that an egg will not stand on either end if unsupported."

merate and Sandstone from the Swiss Alps.—VERTICAL STRATA.

Fig. 4.

Fig. 5.

43. (Fig. 5.) Curved or Contorted Strata of Slate near St. Ann's Head, Berwickshire.-Sir James Hall explains very happily the mode in which these strata were formed. He placed a set of layers of clay under a weight, and then pressed their opposite ends together with such violence as to com

pel them to approach. On the removal of the weight, the layers of clay were found curved after the manner shown in the above engraving.

44. Unconformable Strata.-Our example is chosen from the junction of the Old red sandstone and Silurian schist, at the Siccar Point, near St. Ann's Head, Berwickshire. Its meaning is obvious; while the lower stratum is

Fig. 6.-UNCONFORMABLE STRATA.

inclined or vertical, the upper is horizontal: such combinations are called unconformable. The explanation is easy. The inclined stratum was raised and turned out of its natural position, and then the next was naturally formed horizontally upon it.

45. Other Strata are also distinguished by special names, as, tilted, when suddenly bent up by subterranean force; saddle-back, or anticlinal, when dipping from a common ridge in two opposite directions; and trough, or basin-like, or synclinal, when exhibiting the reverse of the last position, or dipping from opposite directions to a common point. When a stratum comes to the surface and appears it is said to crop out.

CHAPTER III.

THE MEANS OF GEOLOGICAL STUDY-FOSSILS.

Meaning of Fossilization.-By fossilization we mean the study of those relics of vegetables and animals which are found in stratified rocks, and which only cease to appear as we delve down towards the granite.

State of Preservation in which Fossils are found.-Fossils are found in varying states of preservation, of modification, and of almost entire change. They are often broken—a fact that may be ascribed to the turbulence of the actions which accompanied their original inhumation; and often worn, by long rolling against hard surrounding substances. Certain portions, again, of a fossil, will decay, while the rest remains uninjured. The pieces of bivalve shells are thus often discovered apart, through the decay of the hinge ligaments. Mechanical compression produces peculiar effects, as may be witnessed in the compressed ammonites found at Watchet and other places; in the goniates and pectens of Bradford, in Yorkshire; and in the fishes and ichthyosauri of Charmouth. Perhaps the most interesting cases of fossil compressions are found in the shales and gritstone that overlie coal, where the large cylindrical stems of Sigillaria and Lepidodendron are found as flat as paper, when buried between the lamina of shale, depressed elliptically when

lying across the grits, and retaining their original cylindrical figure when standing erect in the rocks.

Chemistry of Fossil Plants.-The chemical phenomena exhibited by fossils are of the greatest importance. We may illustrate these by a brief review of the chief stages of alteration that plants are found to exhibit in passing from their original and living to a fossil state.

1. They are found but little altered, as in the brown coal formations of the Rhine; and in a particular case at Gristhorpe, near Scarborough, among the oolites, where a plant, called by Lindley the Solenites Murrayana, is found flexible, elastic, and with its tissues quite distinct.

2. The plants have become carbonized or bituminized, a very common conversion in the clays of every geological era, and plentifully in what are called the coal formations.

3. The substance of the original plants passes entirely away, by the combination of its elements with the surrounding parts, so that a mere blank remains; but an eloquent blank, for its shape reveals the sort of being that had once occupied the now desolate space. The coarse gritstone near Leeds affords examples of this state.

4. Lastly, the cells of the vegetable structure become filled with extraneous matter, as carbonate of lime-hence the pyrites of Lepidodendron Harcourtii in the fruits of Sheppey; or with silica, and hence the flinty or silicified wood of Woburn.

Chemistry of Fossil Animals.-These exhibit analogous changes. Thus we have

1. Such relics as the scales of fishes, coverings of shell-fish, and bones of vertebrated animals, and which are often found but slightly changed, in some cases even retaining their gelatinous portions.

2. The next step shows to us entire shells, corals, and echinodermata, composed of carbonate of lime and gelatine; the latter substance, in some cases, still partly preserved. From this state we pass, by almost insensible gradations, to that where the organized substances are entirely lost, as in the oolites especially; and there is either left a vacancy, on the sides of which the lost shell has sculptured itself, as it were, for a future memorial before its disappearance; or,—

3. There is a mass of mere stony matter, which also tells the story of what has taken place, by exhibiting on its surface the exact representation of the animal whose being it has absorbed into its own. It is curious to note, that while it was by the absorption of carbonate of lime the vacancies above referred to were formed, it is by carbonate of lime, in many cases, entering in a state of solution, that we find, in other instances, what would have been vacancies are filled up. Silica or flint, and sometimes (but unfrequently) iron pyrites, fill those vacancies.

Relations of particular Rocks and Fossils.-The relations between rocks and the particular fossils they respectively contain may be illustrated by a few examples. In the green sand formations most of the shells and spongiada are silicious. In the oolites the fossils are chiefly calcareous, lime being one of the commonest of these transforming agencies. In the coal formations the fossils are more or less bituminous. Certain tribes-as the belemnites and ostracea-retain their fibrous or lamellar structure in all sorts of rocks.

Local Distribution of Fossils.-Fossils are found on the tops of the loftiest mountains of the Alps and Pyrenees, showing that what was the original

surface of the crust at the time the mountains had been upheaved was carried upwards with them: fossil plants are found at the bottom of our deepest mines. Of course, fossils, generally, are most plentiful on or near the earth's surface, because the formations there are chiefly of a later origin than the stratified rocks which were uplifted in mountain chains. At the depth of a few thousand yards they cease altogether to appear, with the cessation of the appearance of the stratified rocks, in which alone they are found. There are many interesting peculiarities connected with the local distribution of fossils. Some of the ancient limestones about Torquay, in Devon, are composed almost entirely of the remains of animals, chiefly Polyparia and Echinodermata, whose hard parts have been thus, in a sense, preserved. Another fossil species-the Ostrea deltoidea-forms immense continuous beds in what is called the Kimmeridge clays of England and France. They extend for many miles about Weymouth; also in North Wiltshire, and in Yorkshire, in our own country, and about Havre, in France.

Comparison of the Living with the Fossil Creation.-Professor Phillips, some years ago, estimated the numbers of existing (or recent) animals and plants, and of the same in a fossil state, in order to show the proportions of the two. In that estimate the living creation was made to contain about 59,000 plants and 115,500 animals. The progress of discovery shows that we may with safety nearly double the numbers. But the proportions between living and fossil plants and animals are not materially affected, and we therefore append them in the following table :—

Proportion of the Living to the Fossil Creation.

But this table requires to be read with caution. As all these fossils are preserved in beds that once formed the bottoms of lakes or oceans, we cannot expect to find terrestrial plants and animals in them in such numbers as we find the shell-fish and zoophytes. The one class must have been carried thither by accident, such as inundations, &c.; the other naturally belong to it. While, therefore, we have of the first but a very imperfect representation, of the second we possess almost as great an abundance as we could desire. We may conclude this part of our subject by stating that organic fossils bear so general an affinity to existing life, that they may be all ranged in the same great classes, most of them in the same great orders and families, some in the same genera, and but very few-and these only in the latest strata in the same species.

Division of Fossils.-By the various opportunities thus indicated we are enabled to divide all fossil relics into

1. Petrifactions.

2. Bituminizations.

3. Metallizations.

4. Marks of vital action.

Petrifaction is the process by which stony matter, in a state of solution,