i86 



RELATION OF TRAP 



[Ch. XXIX. 



Fig. 638. 



A striking example of tliis structure occurs in a resinous trachyte or 

 pitchstone-porphyry in one of the Ponza islands, which rise from the 

 Mediterranean, off the coast of Terracina and Gaeta. The globes vary 

 from a few inches to three feet in diameter, 

 and are of an ellipsoidal form (see fig. 638). 

 The whole rock is in a state of decomposi- 

 tion, " and when the balls," says Mr. Scrope, 

 " have been exposed a short time to the 

 weather, they scale off at a touch into nu- 

 merous concentric coats, like those of a 

 bulbous root, inclosing a compact nucleus. 

 The laminae of this nucleus have not been 

 so much loosened by decomposition ; but 

 the application of a ruder blow will pro- 

 duce 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. 

 Sometimes 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 con- 

 nected with changes of temperature during 

 the cooling of the mass, as will be pointed out in the sequel. (See chaps, 

 XXXV. and xxxvi.) 



Globiform pitchstone. Chiaja di 

 Luna, Isle of Ponza. (Scrope.) 



Relation of Trappean Bocks 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 analogy 

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

 and the lavas of active volcanoes, it seems diflScult 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 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 subterranean igneous action were contrasted with loose cones of scorise, 

 tuff", and lava, or with narrow^ streams of lava in great part scoriaceous 

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

 Etna, the resemblance seemed remote and equivocal. It was, in truth, 



* Scrope, Geol. Trans. 2d series, vol. ii. p. 205. 



