Fig. 33.

The vertical lines a, fig. 33, represent the basalt partly incumbent in the unconformable position, on the horizontal strata c c. At b b, the basalt has forced up the sandstone, and passed between-its strata, two of which it has entirely separated from their fellows, forming between. them sheets of its own matter.

Dykes of basalt form angles with the horizon of every quantity. Sometimes they are nearly or quite perpendicular to the horizontal strata through which they pass. In some instances a large vein is pierced by a smaller one, which passing through its middle, divides it into two parts. The adjoining cut from Dr. Ure, represents such an instance.

Fig, 34, a a, represents the great basaltic dyke, passing through calcareous sandstone, and b, a small vein of the same matter, by which it is pierced through the line of its axis. The latter is singularly undulated, somewhat like

the zig-zag line of an electric shock passing through the atmosphere.

Although, in most instances, the trap veins pass from below, towards the surface, still there are instances where they descend from the surface into fissures beneath. This is among the more rare and remarkable phenomena which this interesting rock exhibits.

This example occurs in the island of Sky, and is described by Dr. Macculloch. The basaltic veins traverse strata of sandstone, in a vertical direction, and parallel to each other. Fig. 35.

They appear to descend from the mass on the surface, and are so numerous, in some places, as nearly to equal, taken collectively, the mass of rock through which they pass. Sometimes six or eight veins occur within the of 150 feet, and their aggregate magnitude is apparently sixty or seventy feet. Their average breadth is about ten feet, though they vary from five to twenty feet.

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It is certainly very difficult to account for the manner in which nature performed this work. Were these veins only an inch or two thick, we might suppose that the melted trap ascended by the large middle vein, seen in the drawing, and having fissured the sandstone by its great heat, descended again by the apertures thus produced; but the fissures are much too large for such a supposition.

It is supposed that every basaltic dyke terminates in a mass of basalt below the surface, and, therefore, that ba

saltic rocks, resting on the surface, are connected by the dyke or fissure through which they were thrown up, with that part of the mass which still remains beneath the earth. So that these dykes are necks passing through the crust of the earth and connecting the two masses. Where dykes do not reach the surface, of course they are only connected with the lower masses.

Columnar Basalt. All the members of the trap family occasionally assume the form of columns, more or less perfect, but in this respect, basalt excels the others.

These columns are formed by a natural division of the whole mass of basalt in a vertical direction. They vary in the number of their angles, from three to eleven or twelve, the medium polygons having from five to seven faces. These are often perfectly regular, the angles being sharp and well defined, and the faces plane and smooth, as represented by the annexed cut, fig. 36.

Fig. 36

In most cases, when standing in their original positions, their sides are in contact, or so little separated as barely to admit the infiltration of carbonate of lime; a striking difference, as observed by Dr. Macculloch, between them and the irregular prisms which result from the cracking of dried clay, and showing that the nature of the process by which these divisions are made, (whether crystalline or not,) are entirely different from each other.

The columns are sometimes continuous, at others jointed, either obliquely or at right angles; occasionally, also, they are fissured without the appearance of regular joints.

The appearance of a six-sided basaltic column, regularly jointed, that is, consisting of short prisms laid on each other, is represented by fig. 37. It is not common, however, that the prisms are as regular, with respect to length, as here represented, the joints being more commonly repeated at intervals, varying from a few inches to several feet.

Fig. 37.

In their lengths, these columns also differ exceedingly. In the island of Sky are some which are 400 feet long, while others are only an inch in length. In diameter some are several feet, while others are less than an inch.

In exposed situations the prismatic blocks represented by fig. 37, lose their angles by the action of the weather, and become globular, but still retain their columnar position as shown by fig. 38.

It must not be understood that basaltic columns preserve their vertical positions, as usually represented by the drawings of Staffa and the Giant's Causeway, these being rare instances, both with respect to position and height. These columns are placed in every manner, from the horizontal to the vertical angle, though attracting most attention in these latter cases, from their resemblance to the efforts of architecture.

Trap rocks often form mountains of considerable height and sometimes spread over large districts of country. The island of Sky, on the western coast of Ireland, is one continuous mass of erupted rock, fifty miles long and twenty broad. With respect to the elevation of trap mountains, the following are examples. Tinto in the district of Clyde, is 2036 feet high. Benmore in the island of Mull, 3097. Salisbury Craig, 550, and Arthur's Seat, 800 feet; the two last near Edinburgh.

On this side of the Atlantic, Mount Holyoke in Massachusetts is 830 feet above the Connecticut, and 900 feet above the level of the sea. Mount Tom, on the opposite side of the river is still more elevated, being nearly 1000 feet high.

In the valley of the Connecticut, the mural side of the greenstone formations, is generally, and perhaps always towards the west, in which direction the precipices are often nearly perpendicular; while towards the east, the ascent is commonly quite gradual. Whether this fact has

been observed of the greenstone of other countries, we do not know.

Who can conceive of the mighty power which forced these enormous masses from the bowels of the earth; or the awful scenery which was exhibited, when they were poured forth in the form of red hot lava? for there is no doubt but this was the manner of their production.

In most instances, basaltic and greenstone mountains present the form of rounded outlines, with occasional precipices on one or more of their sides.

The configuration of the basaltic columns of Staffa, represented by fig. 39, is peculiarly striking on this account. A part of the mountain has fallen down, in the form of pillars of various dimensions, leaving the others standing in fair view, and preserving a high mural face of great elevation, composed entirely of columnar pieces, touching each other.

The rounded form of the massive cap, which surmounts these pillars, presents the outline common to basaltic hills.

Protrusion of Greenstone. Although greenstone strictly belongs to the trap family; and passes by insensible degrees into basalt, still there have been detected but few instances, where it has protruded through superincumbent rocks so as to exhibit the fact to the eye of the geologist. The diagram fig. 40, from Prof. Hitchcock's Geology of Massachusetts, shows such a case.