brittle, breaking generally into cuboidal lumps. There are three marked varieties: (1) Caking coal, which runs together into clinkers or cinders: the Newcastle coals are of this kind. (2) Splint or Hard coal, common in Scotland, not easily broken or kindled. (3) Cherry coal, which is lustrous, easily breakable, quickly kindled, and does not cake. To this variety most of the Staffordshire coals belong.

Anthracite or Stone-coal is very hard and heavy, with a glossy lustre, and a more completely mineralized appear

ance.

The transition from Lignite to Anthracite by the gradual removal of the oxygen is shown in the following table:

Bitumen or Asphalt is a light, brittle, black, or nearly black mineral, some varieties much resembling cannel coal, but differing in being soluble in turpentine and in melting under the influence of heat. It contains much more hydrogen than coal, and belongs to the group of substances known as Hydrocarbons. It occurs in lenticular lumps and masses among shales and flaggy sandstones, and sometimes forms layers of considerable extent, as in the island of Trinidad (West Indies).

Petroleum is a liquid hydrocarbon, some varieties being thin and clear, others thick and viscid, like tar. Being liquid, it is not confined to the rocks in which it was first generated, but makes its way along cracks and fissures, and even rises to the surface in the manner of an artesian spring. Hence it occurs in rocks of all kinds and ages. Some shales and sandstones hold large quantities of it,

and shale which is so saturated as to yield oil on slow distillation is known as oil-shale. A good oil-shale may be known by the peculiarity that parings made with a knife curl up as they are cut. Oil-shale is generally rich in organic remains, especially of fishes and molluscs, and it seems probable that in most cases the oil has been derived from the decay of these creatures. The richer shales yield from 30 to 40 gallons of mineral oil per ton of shale.

THE

CHAPTER V.

STRATIFICATION.

HE Formation of Strata.-The descriptions given in the first part of this book of the manner in which deposits are formed in the rivers, lakes, and seas of the world are really descriptions of the formation of strata, and we have throughout assumed that the reader has a general comprehension of what is meant by lamination and stratification, so that it is not now necessary to enter into any elaborate definition of these terms.

It is sufficient, therefore, to say (1) that stratification is the arrangement of deposited matter in regular layers or strata; (2) that lamination is a structure possessed by certain strata which separate into still thinner layers or laminæ, each of which is a separately deposited film of sediment.

Laminæ and strata are formed in exactly the same way, by the deposition of successive layers of material, and the difference between them is simply that of the thickness accumulated between the pauses of deposition; these pauses being marked by the planes along which they separate.

All stratified rocks, whether coarse or fine in texture, split into beds or strata, which may be of any thickness from an inch to several feet; but it is only rocks of fine texture which split into laminæ. In some cases fifty or sixty separate lamina may be counted in the thickness of an inch; in others the laminæ are only discernible as bands of colour, or as forming a kind of grain in the rock, so that it splits most readily in a direction parallel to the planes of bedding.

These general facts of lamination and stratification are

represented in fig. 88, where seven different beds succeed one another in regular order; the dotted beds being meant for sandstones, the lined beds for shales, and the plain beds at the top for limestones. In the limestones no lamination is visible; in the sandstones it is just visible by the arrangement of the component particles along lines which produce a definite "grain"; lastly, the beds of shale are fissile or divisible into separate laminæ.

Since in one bed or stratum of shale there are a certain number of laminæ, say, for instance, it is composed of fifty such laminæ, and since each of these was separately deposited, it follows that so many separate acts of deposition

7

6

5

4

3

2

1

A

m

Fig. 88. Lamination and Stratification.

took place in the making of that single stratum, and that its production was a process involving a considerable amount of time.

Lamination does not depend on the fineness of the material, nor altogether on the presence of flaky minerals such as mica, but chiefly on the manner in which the material was spread out. The want of lamination may be due to the continual subsidence of fine sediment for a long time without any pause in the deposition, so that the whole forms an inseparable bed or mass of clay or marl; or it may be due to the rapid accumulation of a large quantity of sediment at once, the result, perhaps, of a single flood, as in the case of some beds of sand or gravel.

Length of the Intervals between Beds. The intervals or periods of non-deposition marked by the planes of separation between beds must vary considerably in

length. The interval between the formation of two contiguous beds of similar lithological character is not likely to have been so long as that between beds of different lithological character. In a series of sandstones, for example, the pauses between the deposition of the successive beds may not have been longer than the intervals between successive tides, and it is clear at any rate that the same physical conditions which favoured the deposition of the first beds were continued during the deposition of the succeeding beds.

On the other hand, in such a series of strata as those represented in fig. 88, where each superimposed bed is different to that on which it rests, the intervals between the beds represent pauses during which some change took place in the surrounding physical conditions. Thus, in the case of a shale resting on a sandstone, we must suppose some alteration to have occurred either in the velocity of the currents, or in the geography of the district. The change may have resulted from a diminution in the strength of the current, so that it could only transport mud to the place where it previously brought sand; or from the introduction of a fresh current carrying mud only instead of sand; or, lastly, from the depression of the land whence the supply was obtained, so that the distance from the shore was increased, and only finer sediment could be deposited.

In the case of a sandstone succeeding shale, we should assume the reversal of one of the causes above mentioned, either an increase in the velocity of the current, or the ingress of a sand-bearing current, or the general elevation of the coast, which would probably change the set of the currents altogether. A limestone resting either on shale or sandstone would lead us to conclude that similar changes were carried on to a still greater extent, till they resulted in the cessation or diversion of all mud-bearing currents, and a long period of rest or slow subsidence ensued, during which the limestone was slowly formed by the accumulation of shells and other calcareous structures secreted by the various animals which lived in the clear waters.

A shale overlying a limestone is, of course, the reverse of the case just mentioned, and the invasion of clear water