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TRAP-ROCKS. 



TRAP-ROCKS; an important class of rocks in 

 geology, which derive their name from the Swedish 

 word trappa, a stair, because they frequently divide 

 into regular forms, resembling the steps of stairs. 

 These rocks vary in texture, from an apparently 

 simple rock to a confusedly crystalline compound, 

 in which crystals of feldspar are disseminated. The 

 predominant substance in the members of the fam- 

 ily is a simple rock, of which indurated clay (wacke) 

 may be placed at one extreme, and compact feld- 

 spar at the other, the intermediate members being 

 claystone or clinkstone. 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 varia- 

 tions in the fundamental character. It often ap- 

 pears as if quartz, feldspar, and hornblende, com- 

 posed the mass, and various circumstances deter- 

 mined 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 feldspar, while the quartz rarely 

 predominates. In other situations, confusedly 

 crystalline compounds have been the result. Quartz, 

 feldspar, and hornblende united form sienite; or 

 feldspar and hornblende, without the quartz, con- 

 stitute greenstone. The compounds occasionally 

 contain disseminated crystals of feldspar, and thus 

 become what are called greenstone porphyries (dia- 

 base porphyroide, French ; Grunstein Ptj-phyr, 

 German). A paste of green hornblende, contain- 

 ing crystals of feldspar, constitutes 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 formation. Such substances are either 

 agates, calcareous spar, or some of the zeolitic mine- 

 rals. From these cavities frequently being of an 

 almond 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 constantly the 

 same, but is liable to vary materially. A trap- 

 rock is sometimes both amygdaloidal and porphyr- 

 itic at the same time. Other minerals besides those 

 above enumerated occur in the trappean rocks, but 

 cannot be considered as forming an essential part 

 of them, with the exception of augite and hyper- 

 sthene, which, with the mixture of either common 

 compact, or glassy feldspar, constitute the augite 

 and hypersthcne rocks. It would be inappropriate 

 to the present article to attempt a notice of the 

 various aspects under which these rocks present 

 themselves. It should, however, be remarked, that 

 the term basalt is applied to substances which are 

 not precisely the same, being sometimes given to a 

 fine compound of augite and compact feldspar ; at 

 others, to a minute mixture of hornblende and com- 

 pact feldspar; sometimes to dark indurated clay- 

 stones ; and finally to a compound of feldspar, augite, 

 and titaniferous iron. The last mixture seems that 

 now most commonly termed basalt. Basalt is pos- 

 sessed 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 contains, as is the fact also with green- 

 stone, passes, when exposed to the air, into a fur- 

 ther state of oxygenation; and they are consequently 



generally covered with a reddish-brown incrusta- 

 tion. 



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 gran- 

 ite, has been observed in a mountain near Chri-ti 

 ania, in Norway. The lower rocks are gneiss, over 

 which occurs dark slate, in which are beds <>t blaek- 

 ish limestone, containing organic remains. These 

 beds are covered by an enormous mass of porphyry, 

 varying in thickness from 1600 to 2000 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 porphyry in enormous masses, often covers the 

 primary mountains of the Andes. They are ar- 

 ranged 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 fre- 

 quently occurs intermixed with, or covering, transi- 

 tion rocks, and basalt is most commonly associated 

 with the secondary strata, which it either cuts 

 through in the form of dikes, or covers unconform- 

 ably. 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 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 sub- 

 stance of the coal is, for several feet, converted 

 into soot. At a 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 thick- 

 ness 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 ex- 

 tends from the western side of Durham to Berwick, 

 in Yorkshire. These dikes are generally harder 

 than the rocks they intersect, and, when the latter 

 are partly decomposed, often remain, 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 occasion cascades, of which 

 there are frequent instances on the river Tees. 

 From these circumstances, 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 occur- 

 rence of trap-rock^ forming isolated caps on distant 

 mountain's, was for a long time considered as oppos- 

 ing 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, nhich 



