620 EELATION OF TRAP, LAVA, AND SCORLE. [Ch. XXIX. 



A fissile texture is occasionally assumed by clinkstone and other 

 trap rocks, so that they have been used for roofing houses. Some- 

 times the prismatic and slaty structure is found in the same 

 mass. The causes which give rise to such arrangements are very 

 obscure, but are supposed to be connected with changes of tempera- 

 ture during the cooling of the mass, as will be pointed out in the 

 sequel. (See Chaps. XXXV. and XXXVI.) 



Relation of Trappean Rocks to the products of active Volcanoes. 



When we reflect on the changes above described in the strata near 

 their contact with trap dikes, and consider how complete is the anal- 

 ogy or often identity in composition and structure of the rocks called 

 trappean and the lavas of active volcanoes, it seems difficult at first to 

 understand how so much doubt could have prevailed for half a cen- 

 tury as to whether trap was of igneous or aqueous origin. To a cer- 

 tain extent, however, there was a real distinction between the trap- 

 pean formations and those to which the term volcanic was almost 

 exclusively confined. A large portion of the trappean rocks first 

 studied in the north of Germany, and in Norway, France, Scotland, 

 and other countries, were such as had been formed entirely under 

 water, or had been injected into fissures and intruded between strata, 

 and which had never flowed out in the air, or over the bottom of a 

 shallow sea. When these products, therefore, of submarine or sub- 

 terranean igneous action were contrasted with loose cones of scoriae, 

 tuff, and lava, or with narrow streams of lava in great part scoria- 

 ceous and porous, such as were observed to have proceeded from 

 Vesuvius and Etna, the resemblance seemed remote and equivocal. 

 It was, in truth, like comparing the roots of a tree with its leaves 

 and branches, which, although they belong to the same plant, differ 

 in form, texture, color, mode of growth, and position. The external 

 cone, with its loose ashes and porous lava, may be likened to the 

 %ht foliage and branches, and the rocks concealed far below, to the 

 roots. But it is not enough to say of the volcano, 



" quantum vertice in auras 

 iEtherias, tantum radice in Tartara tendit," 



for its roots do literally reach downwards to Tartarus, or to the re- 

 gions of subterranean fire ; and what is concealed far below is proba- 

 bly always more important in volume and extent than what is visible 

 above ground. 



We have already stated how frequently dense masses of strata 

 have been removed by denudation from wide areas (see Chap. VI.) ; 

 and this fact prepares us to expect a similar destruction of whatever 

 may once have formed the uppermost part of ancient submarine or 

 subaerial volcanoes, more especially as those superficial parts are 



