TRAP-ROCKS. 
lose a leaf. Hence it is that plants of all kinds 
which are inclined to grow too vigorously receive 
a wholesome check when they are young; that 
fruit-trees are root-pruned, and bear the earlier 
for it; that pine-apples are disrooted to hasten 
their fruiting ; and that many plants which are 
apt to run away and grow too fast are frequently 
removed, and each time with the loss of some of 
their roots. See the article Puanrine. 
TRAP. See the articles Mousn, Rat, and 
VERMIN. 
TRAPA. See Caurrops (WaTER). 
TRAP-ROCKS. An important class of rocks 
in geology, which derive their name from the 
Swedish word trappa,a stair, because they fre- 
quently divide into regular forms, resembling 
| the steps of stairs. 
These rocks vary in texture, from an appa- 
rently simple rock to a confusedly crystalline com- 
pound, in which crystals of felspar are dissemi- 
nated. The predominant substance in the mem- 
bers of the family is a simple rock, of which 
indurated clay (wacke) may be placed at one 
extreme, and compact felspar at the other, the 
intermediate members being claystone or clink- 
stone. In some cases, it forms the whole mass; 
in others, it is mixed with other materials in 
various proportions, producing great diversities 
of aspect, without any material variations in the 
fundamental character. It often appears as if 
| quartz, felspar, and hornblende composed the 
mass, and various circumstances determined their 
union in such a manner as to produce a large 
proportion of the different compounds known as 
trap-rocks, sometimes the hornblende being in 
mass, at others the felspar, while the quartz 
rarely predominates. In other situations, con- 
fusedly crystalline compounds have been the 
result. Quartz, felspar, and hornblende united 
form sienite; or felspar and hornblende, without 
the quartz, constitute greenstone. The compounds 
occasionally contain disseminated crystals of 
felspar, and thus become what are called green- 
stone porphyries (diabase porphyroide, French ; 
Grunstein Porphyr, German). A paste of green 
hornblende, containing crystals of felspar, con- 
stitutes the antique green porphyry (the ophite of 
the French). Some of the trappean rocks are 
often vesicular, in the manner of modern lavas; 
the vesicles, however, being generally filled up 
by some mineral substances, which have been 
infiltrated into them subsequent to their forma- 
tion. Such substances are either agates, cal- 
careous spar, or some of the zeolitic minerals. 
From these cavities frequently being of.an al- 
mond shape, or rather from the appearance of 
their solid contents resembling almonds in form, 
the term amygdaloid has been applied to rocks of 
this description. It must be understood that 
the base, or paste of the amygdaloids, is not con- 
stantly the same, but is liable to vary materially. 
A trap-rock is sometimes both amygdaloidal and 
porphyritic at the same time. Other minerals 
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481 
besides those above enumerated occur in the 
trappean rocks, but cannot be considered as 
forming an essential part of them, with the ex- 
ception of augite and hypersthene, which, with 
the mixture of either common compact or glassy 
felspar, constitute the augite and hypersthene 
rocks. 
It would be inappropriate to the present arti- 
cle to attempt a notice of the various aspects 
under which these rocks present themselves. It 
should, however, be remarked, that the term éa- 
salt is applied to substances which are not pre- 
cisely the same, being sometimes given to a fine 
compound of augite and compact felspar; at 
others, to a minute mixture of hornblende and 
compact felspar; sometimes to dark, indurated 
claystones, and finally to a compound of felspar, 
augite, and titaniferous iron. The last mixture 
seems that now most commonly termed Jdasalt. 
Basalt is possessed of a greenish, or brownish, 
and sometimes of an iron-black, colour. It is 
difficult to break, and possesses a considerable 
degree of hardness. It is fusible into a black 
glass, and is magnetic. The iron which it con- 
tains, as is the fact also with greenstone, passes, 
when exposed to the air, into a further state of 
oxygenation; and they are consequently gene- 
rally covered with a reddish-brown incrustation. 
—The whole family of trap-rocks have, on the 
one hand, a close alliance with volcanic rocks, 
and, on the other, with the more ancient rocks 
of porphyry and granite. The gradation of trap- 
rock, having in some parts a volcanic character, 
into true granite, has been observed in a moun- 
tain near Christiania, in Norway. The lower 
rocks are gneiss, over which occurs,dark slate, in 
which are beds of blackish limestone, containing 
organic remains. These beds are covered by an 
enormous mass of porphyry, varying in thickness 
from 1,600 to 2,000 feet. In the lower part of 
the bed, the porphyry becomes vesicular, and 
changes into an amygdaloidal basalt, containing 
crystals of augite. Basalt, associated with por- 
phyry in enormous masses, often covers the pri- 
mary mountains of the Andes. They are arranged 
in regular columns, which strike the eye of the 
traveller like immense castles in the sky. Por- 
phyritic rocks may, in general, be regarded as 
more ancient than basaltic rocks, as porphyry 
most frequently occurs intermixed with, or cover- 
ing, transition rocks, and basalt is most commonly 
associated with the secondary strata, which it 
either cuts through in the form of dikes, or co- 
vers unconformably. Sometimes it appears to 
have broken the strata confusedly, and to have 
enveloped large portions of other rocks. All the 
trappean rocks give decisive indications of an 
igneous origin, not only in the shapes of their 
masses, but in their action on the adjacent rocks. 
Where basalt is in contact with gneiss, it becomes 
nearly compact, and approaches to the character 
of hornstone; and where greenstone rests on 
sandstone or clay, these rocks have a red and 
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