128 



TRAP ROCKS. 



Trap rocks, on the contrary, are found on the surface in overly- 

 ing, unconformable masses, or are imbedded in other rocks, or inter- 

 sect them, rising like a wall, and breaking the continuity of the strata. 

 Such walls are called dykes. Trap rocks that are imbedded, sel- 

 dom preserve the form of regular strata for any great extent, but are 

 extremely variable in their thickness ; in many instances, they appear 

 to have been laterally protruded between regular strata. These dif- 

 ferent positions of trap rocks are represented Plate III. fig. 2. It 

 is obvious, that these unconformable rocks were formed at a period 

 subsequent to that of the rocks which they cover or intersect. 



As the mineral composition of trap rocks is nearly the same as 

 that of rocks whose igneous origin is now undisputed, we can have 

 little difficulty in admitting, that the overlying masses of trap have 

 been poured over the surface of the conformable rocks in a state of 

 fusion, like streams of lava from recent volcanoes ; with this differ- 

 ence, that they were not erupted from one opening or crater, but | 

 from fissures of great width and many miles or leagues in extent, * 

 and that they were formed under the ocean. I say we can have lit- 

 tle difficulty in admitting this, particularly as such rents or fissures, 

 filled with similar matter to that of the overlying unconformable 

 masses, are often discovered in their vicinity. 



Trap rocks, however, are not unfrequenlly observed imbedded be- 

 tween strata of aqueous formation : here their origin appears more 

 obscure. In many of these instances we may, without difficulty, ad- 

 mit, that these traps rocks were formed by submarine volcanoes, 

 which have poured beds of lava over the limestone ; another bed of 

 limestone may have been subsequently formed over the lava, and this 

 limestone may also have been covered by the lava of a later erup- 

 tion. In this manner the alternation of beds of basalt, or basaltic 

 amygdaloid, with limestone in Derbyshire, may admit of a probable 

 explanation. See Plate IV. fig. 5. e. e. beds of trap between beds 

 of mountain limestone a. a. 



On the southern side of Etna there are several beds of undoubted 

 lava alternating with limestone, as will be more fully stated hereafter. 

 In some instances, however, the basalt or trap has, evidently, been 

 protruded between the strata, after the period when the latter were 

 deposited. 



Before we proceed, it may be proper to remark, that there are 

 certain porphyritic rocks bearing the general character of trap rocks, 

 which are associated with slate rocks, and appear to pass by grada- 

 tion into them. We cannot suppose that they have been erupted 

 like lava, or protruded into slate : they have probably been softened 

 by subterranean heat with the slate in situ ; but from difference of 

 composition, or different degrees of temperature, these beds may 

 have had a greater facility in acquiring a porphyritic texture. A re- | 

 markable instance of the passage of slate into porphyry will be no- 

 ticed hereafter. 



