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or they occupy larger irregular spaces between other rocks, or they have overflowed through craters, and create accumulations after the manner of lava in streams, in plains, or conical heaps.

Where the igneous rock forms a clear and evident filling of a previously existing cleft or fissure, it is termed a dyke or vein. The latter term is, however, more usually confined by many to such as are metalliferous. The irregular disrupting masses are called in German Stöcke (stehende or liegende Stöcke), for which terms there are no precise equivalents in English nomenclature of very general acceptation. Where igneous rocks are accumulated in great extent, and they appear to have filled greater gaps in the earth's crust, they are spoken of as ranges, districts, or tracts. These are sometimes of approximately circular or elliptical shape in their horizontal extension, as may be observed on geological maps. From such ranges, again, there frequently run smaller branches in different directions (ramifications).

Where igneous rocks in a state of fusion have broken through other rocks and spread themselves over the latter, they are said to be overlying. They are either extended longitudinally in one direction in the manner of streams of lava, or they cover broad surfaces, and form extensive fields. In both cases they may afterwards be themselves covered by later rock formations.

The form which the igneous rocks assume above the surface of the ground corresponds little with that of their mass beneath, the geographical outline alone is determined by the latter not the elevation. Very recent igneous rocks, by reason of their volcanic origin, may be of conical shape, as is the case with many basalts, phonolites, or trachytes; but all the older igneous rocks owe their present shape to the transforming influence of long continued weathering and flooding, so that their present appearance depends much more on their individual power of resistance to those influences than upon the shape in which they first made their appearance on the surface of the globe.

The shape of the sedimentary and metamorphic rocks is always flat, or nearly so. Their material was originally deposited on surfaces more or less even, and if inequalities

existed they were filled up, so that at least the upper strata of such deposits are always very regularly flat shaped, or very broadly lenticular. The general shape and extent of these rocks corresponds therefore, more or less, with that of every individual stratum of the same. The conformation of the actual surface in many cases has, however, been much changed by external forces, such as weathering, the action of flood waters, &c.; and, again, the lowest beds of the series may exhibit very great inequalities; even former rents or fissures in underlying rocks may have been filled up by the material of the deposit, so that these fillings of clefts may subsequently assume the shape and character of veins or dykes, in the underlying rock. Such last mentioned cases, however, are rare.

Rents in the earth's crust have come to be filled in very various ways, e. g. by the injection of matter in a state of igneous fusion, by mechanical deposit from above, or by chemical precipitate from solutions. Such fillings are called dykes, veins, or lodes. The term lode is exclusively applied to a metalliferous vein; so also by some geologists is the term vein, but this is not the universal practice. The term dyke is exclusively applied to such as consist of the same material throughout.

Although the sedimentary rocks, as a rule, form extensive flat-lying systems of stratification, yet there occasionally occur irregular accumulations distinguished from the ordinary flat strata by proportionately greater thickness and less horizontal extent, as well as by irregularity of shape. They may have sometimes arisen by filling of

caverns.

The bedding of rocks may be divided into the regular and irregular. The latter is characteristic of the igneous rocks, the former of the sedimentary and metamorphic.

Irregular bedding is in general the consequence of a violent disruption of the pre-consolidated earth's crust. The igneous rocks have forced themselves a path through the existing rocks and filled up the cracks made in the latter by the eruption. These are sometimes, but not always, regularly formed fissures, such as when filled can be called dykes. These violent disruptions are termed intrusions, and when they are of unmistakable character we may conclude with certainty that the intruding rock is

of more recent formation than the one broken through, but not how much more recent.

Regular Bedding, which, as we have said, chiefly prevails in the sedimentary and metamorphic rocks, corresponds with their internal stratification.

The following are some of the phases of bedding:

1. Parallel alternating bedding, or uniform bedding, when two or more rocks alternate with each other in parallel strata, forming a whole system of strata whose general shape is flat or gently swelling. 2. Divergent bedding.-When any set of beds incline in different directions, they sometimes incline towards each other (synclinal), and sometimes they fall away from each other (anticlinal).

3. Overlapping (übergreifend), when one set of strata overlaps the edges of another set of strata. 4. A hollow basin-like form (Muldenförmig).

5. Cloak-like bedding (Mantelförmig), where the strata or beds surround and nearly envelop a central point from which they dip on all sides (quaquaversal dip).

6. Subordinate intermediate bedding, when beds of subordinate size lie in the midst of a larger series of

strata.

The originally regular bedding of the sedimentary and metamorphic rocks has very often been more or less disturbed by subsequent processes, such as the intrusion of igneous rocks, subsidence, &c., and even some of the above-named cases are sometimes only the consequence of some such disturbances. The natural or original position of the sedimentary or the metamorphic rocks and their strata is necessarily the horizontal, or nearly so. If we find any very great variations from the horizontal, these are, as a rule, to be considered as the consequence of disturbance, although the original cause of such disturbance may not always be recognised with certainty.

The following are some of the different kinds of disturbance of bedding:

·

1. Uplifting, by which whole strata or systems of strata frequently appear to have been very strongly inclined from the horizontal direction.

2. Contortion, foldings, bendings.

3. Disruption, breaks (Zerknickung), where the strata appear to have been uplifted in the centre and broken, the two parts dipping from the place of rupture. 4. Displacement or faults (Verwerfung) where one portion of the bed has been uplifted or depressed and bodily separated from the remaining portion. 5. Subversion (Ueberstürzung), where the bed has been entirely overturned, and its position reversed.

From the bedding of rocks we may often, but not always, determine their relative age. The principles to guide us in this are somewhat as follows:

1. Overlying rocks as a rule are more recent than those which they cover.

The only exceptions to this rule are created by subversions, or by obliquely upheaved or intruded igneous masses. Such exceptions, however, are usually easily to be recognised by surrounding circumstances.

2. Intruding rocks are always more recent than those which they have penetrated.

The exceptions to this rule can be only apparent; as for instance, if a steep projecting rock has been surrounded by and come to be imbedded in a later deposit and so afterwards possibly been mistaken for an intruder.

3. Rocks which during their formation have created manifest disturbance of the bedding of other rocks are necessarily in every case younger than those.

From this rule also only apparent exceptions can arise, as when the bedding of a rock may have been disturbed by the decay of the underlying stratum, e. g. the dissolution of rock-salt.

4. The level of a rock alone will not enable us to pronounce on its age, for the oldest sedimentary rocks may by upheaval have been shoved up into the highest level; and as regards the igneous rocks, according to their very nature the oldest and the youngest may be met with in any level.

CHAPTER IV.

GEOLOGICAL FORMATIONS AND GROUPS OF ROCKS.

ACCORDING to the present state of our geological knowledge, we regard a certain class of rocks as the original products of the consolidation of parts of the fused mass of which our planet formerly consisted, and which we still believe to be the substance of the interior of the globe. These original products we term igneous rocks. All other rocks are only secondary products arising from their transmutation, their decomposition, decay or disintegration, and reconstruction.

The igneous rocks are subdivided into two principal groups, the Volcanic and the Plutonic. The volcanic are those which, having been ejected from the interior in a fluid or viscous state, cooled and consolidated at or near the surface of the earth. The plutonic are those rocks which have not reached the surface of the earth in a fluid or viscous state, but solidified at considerable depth, probably therefore under influences of great heat and pressure.

The rocks which we have termed secondary products are likewise divisible into two great classes, the Sedimentary and Metamorphic. The sedimentary rocks being formed from the débris of the igneous rocks, and the metamorphic being the older sedimentary rocks, which, in process of time, and from various causes, have assumed an altered character, have undergone metamorphosis.' Let us take a brief review of these principal rock formations:

1. Volcanic Formations.-The active volcanoes of the

present day furnish us with the best instances of these formations, and the surest data for considering the phenomena of their origin. First we find consolidated lavas of various outward form and