482 TRAP-ROCKS. 
burnt appearance, and a hardness superior to 
what they possess in other places. Where they 
cross the coal strata, and come in contact with 
the seams of coal, the substance of the coal is, 
for several feet, converted into soot. Ata greater 
distance from the trap, the coal is reduced to a 
coke or cinder, which burns without smoke, and 
with a clear and durable heat. At the distance 
of fifty feet from the dike, the coal is found in 
its natural, unaltered state. The thickness of 
trap dikes varies from a few inches to twenty or 
thirty yards. The extent to which they reach 
across a country has seldom been explored beyond 
the mining districts. The longest in England 
extends from the western side of Durham to 
Berwick, in Yorkshire. These dikes are gene- 
rally harder than the rocks they intersect, and, 
when the latter are partly decomposed, often re- 
main, forming vast walls of stone that rise above 
the surface of the ground. They also extend 
into the sea, and give rise to reefs of rocks; and, 
when they cross the beds of rivers, they form 
fords, and sometimes hold up the water and oc- 
casion cascades, of which there are frequent 
instances on the river Tees. From these circum- 
stances, it seems conclusive that basalt and 
greenstone (and the same may be affirmed of the 
other varieties of trap-rocks) were thrown out in 
a melted state, like lava, and poured over the 
surface of the ground. The frequent occurrence 
of trap-rocks forming isolated caps on distant 
mountains, was for a long time considered as op- 
posing the hypothesis of the igneous origin of 
basaltic rocks; but a more attentive observation 
of such districts has established the fact, that 
these isolated caps are parts of continuous beds, 
which have, in remote ages, been excavated by 
valleys, in the same manner as the beds of other 
rocks, which frequently form isolated caps on 
detached mountains. 
The occurrence of thick beds of basalt, divided 
into regular pentagonal or hexagonal columns, 
and disposed in ranges of vast extent and height, 
early attracted the attention of mankind, and 
gave rise to various theories respecting their 
formation. Few countries in the world present 
more magnificent deposits of columnar basalt 
than the north part of Ireland and some of the 
Hebrides. The Giant’s causeway, in the county 
of Antrim, constitutes a small part of a range of 
this description. The promontories of Fairhead 
and Borge, in the same range, are situated eight 
miles from each other. ‘These capes consist of 
various ranges of pillars and horizontal strata, 
which rise from the sea to the height of 500 feet. 
From their abruptness they are conspicuous, and 
form a pile of natural architecture, in which the 
regularity and symmetry of art appear to be 
united with the wild grandeur and magnificence 
of nature. Many of the columns in the ranges 
at Fairhead are 150 feet in height and five feet, 
in breadth. At the base along the shore is a 
wild waste of rocky fragments which have fallen 
castles. 
TREADING. 
from the cliffs, resembling the ruins of enormous 
At the Giant’s causeway, the columns 
rarely exceed one foot in breadth and thirty in | 
i 
so much attended to, 
height. They are sharply defined, and the co- 
lumns are divided into smaller blocks, or prisms, 
of one foot or more in length, which fit neatly 
into each other, like a ball and socket. The 
basalt is close-grained, excepting the upper joint 
of the column, which is often cellular. The 
columns usually have five or six sides; but some 
have seven or eight, and others only three. Beds 
of basalt that are not columnar, in some places 
lie over and also under the columns. The basalt 
of the beds is amygdaloidal. The columns at 
Fairhead are not articulated like those of the 
Giant’s causeway ; but blocks, which are of great 
length, lie flat on each other. 
TRAVELLER’S JOY. See Cruumarts. 
TREACLE. See Mouassus. 
TREACLE-MUSTARD,—botanically Clypeola. 
A genus of handsome, hardy, annual plants, of 
the cruciferous order. The violet-thlaspi species, 
Clypeola ion-thlaspi, was introduced to Britain 
from the south of Europe, in the early part of 
the 18th century. It has commonly a height of 
only 3 or 4 inches; its branches rise from every 
side of the root, and are slender and leafy, and 
generally lie prostrate on the ground ; its leaves 
are small, narrow at the base, and obtuse and. 
broader at the end; its flowers grow in short 
close spikes at the extremity of the branches, and 
are small, yellow, and cruciform, and bloom from 
May till July; and its silicles are compressed, 
orbicular, buckler-like, one-celled, and one-seeded. 
—The wool-bearing species, Clypeola ervophora, 
is white-flowered and about the same height as 
the preceding; and was introduced about 30 
years ago from Spain.—The name treacle-mus- 
tard is sometimes given also to some species of 
erysimums and alyssums. 
TREAD. A bruise of the coronet of the hind 
foot of a horse, inflicted by a blow either from 
the foot of another horse behind him, or from 
the calkin of his own other hind foot. It is 
sometimes so slight as to be easily cured by first 
carefully cleaning it, and next applying a pledget 
of tow dipped in Friar’s balsam or in tincture of 
myrrh; but in other instances it is so severe as 
to involve considerable inflammation and to re- 
quire poulticing for a few days, followed by the 
use of Friar’s balsam, digestive ointment, or a 
solution of alum or of white vitriol. In no case 
must it be neglected; for if not promptly and 
duly attended to, it may give rise to quittor. See 
the articles Quirror and OvER-REACH. 
TREADING. The compressing of sown seed 
into the soil with the feet of man or of some of 
the domesticated animals. It serves the same 
purpose as rolling; and is thought, in some re- 
spects, to serve it much better, at least when 
only common cylindrical rollers are used. “ In 
Flanders, where the minutize of agriculture are 
” says Sir John Sinclair, 
