114 



LYELL'S ELEMENTS OF GEOLOGY. 



Relation of Trap, Lava, and Scoriae. 



Fig. 103. 



Globiform pitchstone. Chiaja di Luna, 

 Isle of Ponza. (Scrope.) 



laminee of this nucleus have not been 

 so much loosened by decomposition ; 

 but the application of a ruder blow will 

 produce a still further exfoliation.* 

 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 struc- 

 ture 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 temperature during the 

 cooling of the mass, as will be 

 pointed out in the sequel. (See 

 Chap. X.) 



Relation of trappean rocks to the 

 products of active volcanos. 



I When we reflect on the changes 

 above described in the strata near their contact with trap dikes, 

 and consider how great is the analogy in composition and struc- 

 ture of the rocks called trappean and the lavas of active volca- 

 nos, it seems difficult at first to understand how so much doubt 

 could have prevailed for half a century as to whether trap was 

 of igneous or aqueous origin. To a certain extent, however, 

 there was a real distinction between the trappean formations and 

 those to which the term volcanic was almost exclusively con- 

 fined. The trappean rocks first studied in the north of Germany, 

 and in Norway, France, Scotland, and other countries, were 

 either such as had been formed entirely under deep 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 subterra- 

 nean igneous action were contrasted with loose cones of scoriae, 

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

 riaceous 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, colour, mode of growth, and 

 position. The external cone, with its loose ashes and porous 



* Scrope, Gcol. Trans, vol. ii. p. 205. Second Series. 



