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CHAPTER IX. 



ON UNCONFORMABLE TRAP ROCKS AND BASALTIC DYKES. 



Different Positions of Trap Rocks, as overlying, imbedded in, or intersecting oth- 

 er Rocks. — Varieties of Trap Rocks. — Porphyr}'-, Porphyritic Trap, Sienite, 

 Greenstone, Clinkstone, Basalt, Amygdaloid, and Wacke. — Passage by Grada- 

 tion into each other, and into Volcanic and Granitic Rocks. — Remarkable In- 

 stance of this Passage at Christiania in Norway. — Mountains of Porphyritic 

 Trap and Clinkstone with deep Craters. — High Stile, Cumberland, Cader ldris, 

 Monmouthshire. — Basaltic Dykes: Extent of the Cleveland. Basalt Dyke. — Iso- 

 lated Caps of Basalt. — On inlerstratified Basalt. — Remarks of Professor Sedg- 

 wick on the Protrusion of Basalt between regular Strata. — On columnar Ranges 

 of Basalt. — Organic Remains enveloped in Basalt. — Remarkable Basaltic Dis- 

 tricts in Europe and America. — Experiments on Basalt. — Theory of Werner. 

 On the relative Age of Trap Rocks, 



The class of rocks about to be described in (he present chapter, are 

 extremely interesting to the geologist, as they present him with dc' 

 cided indications of their origin and mode of formation. They not 

 only " reveal the secret of their birth,^^ but, from their close alliance 

 to many of the most ancient primary rocks, they disclose the opera- 

 tions by which a large portion of the earthh surface was consolidated, 

 in the most remote geological epochs. Many of the trap rocks are 

 so similar in structure and composition to the products of active voU 

 canoes, and to beds of lava erupted in our own times, that we may be 

 said to see the very cause in operation, by which they were formed. 

 Many of the trap rocks are also so similar in structure and compo- 

 sition to some of the most ancient primary rocks, that we can scarcely 

 doubt respecting their having had the same origin, though they may 

 have been consolidated under different degrees of heat or pressure, 

 and with different attendant conditions. The name Trap is derived 

 from the Swedish word trappa, a stair, and has been given to rocks, 

 of this class, because many of them divide into regular forms resem,- 

 bling the steps of stairs. Whether the term, in its literal sense, is 

 well chosen as a generic name, may be doubted; but, taken metaphor- 

 I ically, it is extremely appropriate, as these rocks offer a series of 

 gradations or steps, over which the geologist may safely travel in his 

 speculations, from the lava of Etna, to the granite of the Alps. 



To obtain a correct knowledge of trap rocks, the student should 

 first acquire a clear idea of their position. When primary and trans- 

 ition rocks form distinct beds, they are generally arranged conforma- 

 bly, or, in other words, the upper beds are moulded upon the lower, 

 and have the same elevations and depressions, as represented Plate 

 III. fig. 1. 



