different effects of fissures on plastic, and rigid strata; on those which yield most easily to a perforating agent, and those which obstruct and turn it aside, are presented in some of the veins of the Western Islands, as in Lunga, and elsewhere. As the veins of trap are most frequently of large dimensions, so they are in general, either vertical, or inclined at high angles to the horizon. Thus they probably indicate by both circumstances, the great depths at which the parent masses lie, and the nature and place of the force which produced the fissures. They occur also often under low angles.

Instead of breaking across the planes of the strata, these granite and trap injections sometimes spread out between them, forming in the case of the quickly consolidating granite, thick masses, and in that of the more fusible trap, coulees, or extensive lamina. A hasty view of such appearances has given rise to the notion of trap and granite being stratified.

Many chemical and

makasim brk erung 28 mechanical changes

occur in the including strata at the place

where they are tra

versed by veins, whe

ther these be of trap h or granite. The figure on the margin exhibits gneiss, shifted by a granite vein in the island of Col.

"As it is apparent," says Doctor Macculloch, "that granite has been in a state of fluidity beneath

ORIGIN OF TRAP ROCKS.

119

the strata, and that during this state these have been elevated, in an irregular manner, it is easy to account for the irregularity of its general surface, or for the partial way in which it is found distributed on the earth's superficies. The consequence of the unequal elevation of the strata, was to produce those interior inequalities that have been filled by the yielding mass which was the immediate cause of that fracture, and the concomitant of the force exerted. The production of the veins is another obvious consequence of the fractures or discontinuities formed by the displacement of the strata. It must be remembered, however, that the actual appearance of granite at the surface of the earth is in most cases the consequence of another train of effects, consisting in the waste of those parts of the strata with which it was once covered; a waste of which the whole globe bears unquestionable evidence. From the progressive state of that waste, it follows that the apparent quantity of granite on the earth must be constantly increasing, although itself subject to decay; and if it really be the basis (or foundation) of all the stratified rocks, it is possible to conceive that the higher parts of the earth might at some future but exceedingly remote period, contain only granite."

.

We must seek the origin of the trap rocks in the same regions that produced granite. That this is really their source is proved by the masses that lie beneath or among the strata, by the depth and magnitude of their veins, and by the marks of force which accompany their lines of contact with the strata.

If any further doubt could exist, it

would be removed by the phenomena of voicanoes. The substances which these produce are not only strictly analagous, in all their essential characters, to some of the trap rocks, but often undistinguishable. The variations that appear, admit of an easy explanation from obvious causes of difference. These rocks having passed through the strata flow over them in certain cases; while in many others there is reason to suppose from the effects following the earthquakes that accompany them, that they have intruded among the strata beneath the surface, so as to have produced those well-known permanent elevations of the land, found in volcanic countries. That they elevate the superficial strata, is also fully proved by the phenomena attending the volcanic Coral islands.-See Book iii. Chap. ii. Sec. i.

It is in the deeper regions of the globe, therefore, that we must seek the origin of trap; where we found that of granite. These substances are essentially of the same nature, but they have been produced at distant intervals. In accounting for the present superficial position of trap we are provided with two resources; that of its flowing out in the manner of lava, so as to cover the strata, and the final removal of these, so as to leave bare that which was once concealed beneath them.

In the district of Morven, a mountainous mass of trap, rising to twelve or fifteen hundred feet, meets a similar mountain of gneiss, in a line not far deviating from the perpendicular: its base being lost beneath the sea. Here the gneiss reposes on, or meets the trap, precisely as it would meet a mass

STRATA ELEVATED WITHOUT ERUPTION.

121

of granite, and is in the same manner disturbed at the line of junction. This trap mass is indeed connected with a portion that covers secondary strata, and which may be considered as its effused portion. But this part is fast wasting away; and the time may arrive when the trap of Morven shall present all the geological appearances of granite. Had it accidentally possessed the granitic mineral character of some of the syenitic traps of Sky, it would then be supposed an unerupted rock, and a true granite by the Neptunists.

Similar appearances occur in Sky, where masses of trap that seem interminable downwards, pass through the secondary strata, just as granite transpierces the primary. One of these sheets is many miles in diameter; and did neither Sky nor any other district preserve the vestiges of erupted and overflowing trap, it might here also be argued that such matter could not have been ejected from below.

There is in Sky, a mass of trap, indefinite in depth, and of a conoidal form, in a fair, and deep section : It is covered by the secondary strata, which are so bent over it, as to be conformed to its shape. Here is a case exactly analogous to that of an apparently unerupted granite. When in the progress of waste, its summit shall become naked, it will present the semblance of unerupted trap, with the strata conforming to it on every side, just as the primary strata are found to flank mountains of granite.

The volcanic elevations of strata without eruption, present analogies to the protrusions of granite in mountain ridges, carrying up the primitive strata without bursting through them, so as to overflow.

In fact, the thickness and flexibility of these primitive strata afford a sufficient reason for this result. Even the rigid sandstones of Sky have receded before the pressure of the trap; and much more readily would the pliant primary schists, yield before granite.

The admirable crystalline structure of granite, proves it to have lain longer under the influence of heat than the trap rocks. This structure can be produced in our laboratories, by prolonging the semifluid state of fused traps; a condition to which volcanic rocks are liable, from slow cooling in mass. Where granite has been actually erupted, the rocks which it involved may, by long exposure to its action in the ignited state, have been converted into its nature. "An instance occurs in Cantyre, as well as on the continent of Europe," says Dr. Macculloch," where the gradual conversion of a schistose rock into porphyry, under similar circumstances, is proved in a most unquestionable manner."

That granite has overflowed the strata, like trap, is rendered probable by the fact of granite lying above shell-limestone, in Norway, as observed by Von Buch. No witness can be less liable to suspicion than the person who originally resisted the theory, which his own statements have demon. strated to be true.

"To limit the term granite,” says Dr. Macculloch, "to the sole compound of quartz, mica and felspar is merely to abuse a mineralogical term, for the purpose of evading a geological inference. In a geological sense, every rock must be considered a granite, which, whatever its composition may be,