The diagram Fig. 19, from Dr. Macculloch, represents the appearance of these rocks. "The granite at this locality" says Mr. Lyell, "often sends forth so many veins as to reticulate the lime stone and shist, the veins diminishing towards their termination to the thickness of a leaf of paper, or a thread. In some places fragments of granite appear entangled as it were, in the limestone, and are not visibly connected with any larger mass; while sometimes on the other hand, a lump of the limestone is found in the midst of the granite;" a, granite b, limestone, c, argillaceous shist.

The ordinary color of the limestone at Glen Tilt is lead blue, and its texture large grained; but where it approximates to the granite, particularly where it is penetrated by the smaller veins, the crystalline texture disappears, and it assumes an appearance exactly resembling hornstone. This change was undoubtedly produced by the heat of the intruding granite.

These facts and circumstances are considered sufficient to show the igneous origin of granite, though an abundance of others of a similar nature might be adduced from authors.

All the older geological writers believed that granite was the primitive rock of our globe, and the one on which

all others reposed. They also considered this rock as every where of a similar age, the idea of successive formations of granite having never until recently been advanced. These opinions were founded on the general facts, that this rock lies beneath all others, and that it contains no organic remains, which facts even at the present day, we must acknowledge to be generally true. More extensive observations have however shown many exceptions to these facts, there having been discovered instances where granite not only penetrates through, and reposes on stratified rocks, but also where the rocks invaded by it contain organic remains. Thus Dr. Macculloch describes a considerable mass of granite in the Isle of Sky, which is incumbent on limestone, and shale. The limestone at some distance from the granite contains shells, but in its immediate vicinity, no shells appear, the limestone being converted into pure crystalline marble. This change, as well as the destruction of the shells, is attributed to the heat of the granite at the time of its protrusion.

In different parts of the Alps, similar phenomena occur, where according to the observations of Beaumont, and others, granite is seen penetrating through secondary strata, which contain belemnites and other fossil organic remains.

In Norway, also Von Bush discovered a mass of granite overlying a bed of secondary limestone, containing a variety of fossil shells.

These and other instances of the kind, must however be considered as exceptions to a general rule, there being no doubt, but the granite which universally forms the deeper portions of the crust of our globe, is the eldest of our rocks.

DIFFERENCE BETWEEN IGNEOUS ROCKS:

After having shown that granite, as well as greenstone is an igneous rock, the inquiry naturally arises why these two rocks differ so widely in appearance, if indeed they have had the same origin? This is a question which our present knowledge does not enable us to answer with any degree of certainty, nor indeed do geologists profess to do more than offer plausible conjectures to account for these differences.

The composition of greenstone is hornblende and felspar, that of granite is felspar, quartz, and mica. The crystals in the greenstone are commonly small, often too minute to be distinguished by the naked eye, while those of granite are generally of considerable size, often many inches in diameter.

Some geologists have supposed that the difference in the size of the crystals, might be accounted for by the difference in the time of cooling, since chemistry in some instances has shown, that the same materials will form large crystals when cooled slowly, and small ones when cooled suddenly. It has been conjectured therefore, that the trappean rocks were erupted under the sea, and that the pressure of the water, and the rapid abstraction of the heat, by its agency has caused the difference in texture. But if we admit that the granite was fused at a greater depth and in larger quantities, and account for the difference of texture on these conditions, still it is difficult to conceive why such conditions should produce such changes in the compositions of the two rocks, the greenstone containing little or no mica or quartz, while the granite contains only an occasional portion of hornblende.

If we compare granite, and the varieties of trap with the volcanic products of the present time, or with those of ancient, extinct volcanoes, we shall find in general little analogy, either in appearance or composition between them. No volcano either ancient or modern, has ever been known to emit either granite, or trap; though the latter and some volcanic products have considerable affinity.

It is possible that future observations may throw light on this subject, but at present though geologists generally agree that granite, trap and lava were all once in a state of fusion, yet no one has given any satisfactory theory to account for the differences they present in appearance, texture and composition.

TRANSITION OR INTERMEDIATE ROCKS.

Next in order to the primitive are the Transition rocks. The term transition comes from the Latin transitio, in reference to their removal or change of place. These rocks are above the primitive on which they rest.

This formation is composed of the larger fragments of all the primitive rocks, consolidated into continuous masses. The manner in which the transition rocks were formed, appears to be sufficiently obvious. At the time when the waters were gathered into one place, to form the sea, or when the primitive rocks were thrown up from the ocean, the disruptions and dislocations consequent upon these mighty movements, reduced the highest parts of the primitive to fragments, which falling down upon the sides of the mountains, covered them with their ruins; and these becoming agglutinated by the pulverulent cement, produced by the friction of these fragments, formed the rocks in question.

In the course of their consolidation, organized beings of the lowest orders, such as sea shells, falling in their crevices were there embedded; and thus it is proved that these rocks were formed after the creation of organized beings. That they were formed next after the primitive rocks, is proved by their lying immediately on them.

The rocks belonging to this class are Greywacke, Transition Limestone, Slate and Sandstone.

Greywacke. This uncouth word, which we have borrowed from the Germans, the French Geologists have exchanged for the term traumate, which signifies fragment

ary.

Greywacke is a slaty formation, which includes the fragments of many other rocks. These fragments vary in size, from that of the head to the smallest grains. Sometimes it consists almost entirely of rounded pebbles, cemented together by sand and oxide of iron. It is then called conglomerate, and no longer retains its slaty character. When the grains are small, and it is stratified, it becomes slate; and when not stratified, it passes into sandstone. The Rhode Island coal mine is in a greywacke formation.

Transition Limestone. This is an abundant rock, being that which is employed in making quick-lime for mortar, and also in many countries, as a building stone. Many of the common variegated marbles belong to this formation. Some specimens are finely colored, and bearing a high polish, form beautiful slabs for tables and fire-places.

This rock sometimes underlays large sections of country, and in other instances rises into extensive ranges of mountains. The great caverns which are described as existing in different countries, and which often contain the remains of animals, are of this class.

Some transition limestones contain abundance of marine organic remains, and hence must have been formed under the ocean. In other instances, no fossil relics are found, but the rock is composed of angular, or waterworn fragments, consolidated by a calcareous cement. The presence of such fragments will always distinguish the transition from the secondary limestones.

In England and Wales, this is a very extensive and important formation, and contains not only vast quantities of organic relics, but various metallic ores. "In Derbyshire," says Mr. Bakewell," where the different beds of limestone have been pierced through by the miners, the average thickness of the three uppermost, is 160 yards; the beds are separated by beds of trap, or basalt, resembling ancient lavas."

Slate. Clay slate, although often associated with primitive rocks, as already noticed, is also found with those of the transition class. But we have already given a sufficient description of this rock.

Porphyry. This is also, sometimes a transition rock, being so considered when it is found associated with rocks of this class.

Sandstone. This rock, as its name indicates, consists chiefly of sand, cemented into a solid form. It often contains water-worn pebbles, angular pieces of other rocks, as granite, fragments of slate, nodules of quartz, &c. being evidently made up of the ruins of former rocks. Its color is commonly red, owing to the oxide of iron it contains, and which serves as a cement to the grains of sand of which it is composed.

Sandstone, by an uninterrupted continuity, passes into greywacke. The only difference appears to be, that the latter rock is commonly stratified, and of a darker color, not having, like the sandstone, a tinge of red. Where the greywacke is not of a slaty structure, it becomes sand

stone.