c rocks, resting on the surface, are connected by the e or fissure through which they were thrown up, with part of the mass which still remains beneath the earth. hat these dykes are necks passing through the crust e earth and connecting the two masses. Where dykes ot reach the surface, of course they are only connected the lower masses.

olumnar Basalt. All the members of the trap family sionally assume the form of columns, more or less ect, but, in this respect, basalt excels the others.

'hese columns are formed by a natural division of the le mass of basalt in a vertical direction. They vary he number of their angles, from three to eleven or ve, the medium polygons having from five to seven 3. These are often perfectly regular, the angles being 'p and well defined, and the faces plain and smooth, as esented by the annexed cut, fig. 36.

S.

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 cracking of dried clay, and showing that the nature the process by which these divisions are made, ether crystalline or not,) are entirely different from nother.

'he columns are sometimes continuous, at others jointeither obliquely or at right angles; occasionally, also, - are fissured without the appearance of regular

ts.

The appearance of a six-sided basaltic column, reguy jointed, that is, consisting of short prisms laid on other, is represented by fig. 37. It is not common,

-ever, that the prisms are as regular, with respect to th, as here represented, the joints being more comly repeated at intervals, varying from a few inches to -ral feet.

Fig. 37.

Fig. 38.

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 nd of Mull, 3097. Salisbury Craig, 550, and Arthur's t, 800 feet; the two last near Edinburgh.

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

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

18

observed of the greenstone of other countries, we do know.

Who can conceive of the mighty power which forced e enormous masses from the bowels of the earth; or wful scenery which was exhibited, when they were 'ed forth in the form of red hot lava? for there is no t but this was the manner of their production.

Fig. 39.

A

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

he configuration of the basaltic columns of Staffa, repnted by fig. 39, is peculiarly striking on this account. art of the mountain has fallen down, in the form of rs of various dimensions, leaving the others standing, ir view, and preserving a high mural face of great ation, composed entirely of columnar pieces, touching other.

cap,

he rounded form of the massive which surmounts e pillars, presents the outline common to basaltic hills.

Protrusion of Greenstone. Although greenstone strictelongs to the trap family, and passes by insensible dees into basalt, still there have been detected but few inces, where it has protruded through superincumbent is so as to exhibit the fact to the eye of the geologist. diagram fig. 40, from Prof. Hitchcock's Geology of sachusetts, shows such a case.

"The rocks th agency, vers to e turbance found les greenston and gran noticed.

of green

sort. T

dip of the

east. B is consid

The qua mile nor

The protrusion," says Prof. H., "of the unstratified s through the stratified ones, by internal igneous cy, now admitted by most geologists, has led obserto examine carefully for evidences of mechanical disance, near the line of contact. They have, I believe, d less proof of such disturbance by the intrusion of enstone, than in the case of the older rocks, as sienite granite. Every such case, therefore, deserves to be ced. If I mistake not, the following sketch of a vein. greenstone in argillaceous slate, is an example of this

The dyke is about ten feét thick, and the general of the layers of slate in the quarry, is about 30° south. But as shown in the figure, near the greenstone it onsiderably curved upwards in the contrary direction. e quarry, where the example occurs, is about half a e north of the powder house in Charlestown."

MINERAL VEINS.

Metallic veins appear originally to have been fissures, en passing through different beds of rock, and which re subsequently filled with metallic ores. These veins st therefore be considered as subsequent formations to rocks through which they pass. When, however, a n is found in only one bed of rock, the vein may have n formed and filled at the time when the rock was condated.

When mineral veins occur in considerable numbers in tract of country, they maintain a general parallelism, if all the fissures to which they owe their origin, had en formed at the same time, by some common cause.

The absoluto antiquity of voins cannot be coniectured

where one vein intersects another, as is often the case, dislocation of the strata, through which the oldest 1 passes, by the contact of the new one, is sufficient to w a difference in their ages.

Teins exist in primitive, transition, and secondary rocks, are most common in the former. The substances

st commonly found in them, are the metals, quartz, caleous spar, barytes, and Derbyshire spar. It hardly d be remarked, that the chief object in pursuing veins, he metals which they contain.

With respect to the depth of metallic veins, nothing but
jecture can be offered. The miners believe that they
ch quite through the earth, but this opinion has no
er foundation than that they never find their termina-
. Indeed, it is believed, that no instance has been re-
led, where the end of a metallic vein has been found.
ey, however, often grow too poor to pay the
expense
vorking; and the difficulty of clearing deep mines of
water, is a frequent reason why they are relinquished
en they would pay a good profit, were they near the
face. Veins are seldom rich near the surface; but
'ease in value at a medium depth, and grow poor again
greater.

Metallic veins often change their metals at different
ths. In France, there are veins which contain iron
ve, then silver, and below the silver, copper;
and one
he Cornwall mines contains zinc, in the upper part of
veins, which becomes rich in copper at a greater depth.
ns often change their dimensions also, being narrow
ome parts, and wide in others. Thus the Dalcoath
e of Cornwall, varies from forty feet, to six inches in
th.

t is a curious circumstance, that where a vein is inter-
ed by a dyke, that the former often divides into two
nches, which unite again before reaching the latter,
after having passed it, separates into several ramifi-

ons.

shows ho ing narr

Thus, fig. 41, b b, is the dyke, and a a, the metallic n, divided at a, but united again before reaching the te, after passing which, it again separates into several ts. The dyke has occasioned a fault, by which the ends of the vein are widely separated. The lower nches are not supposed to terminate as represented in cut, but to unite again and proceed downwards. c c.