Oh. XXIX.] RELATION OF TRAP, LAYA, AND SCORLE. 



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Strata intercepted by a trap dike, 

 and covered with alluvium. 



always of the lightest and most perish- Fig. 692. 



able materials. The abrupt manner in 

 which dikes of trap usually terminate at 

 the surface (see fig. 692), and the water- 

 worn pebbles of trap in the alluvium 

 which covers the dike, prove incoutesta- 

 bly that whatever was uppermost in these 

 formations has been swept away. It is 

 easy, therefore, to conceive that what is 

 gone in regions of trap may have corre- 

 sponded to what is now visible in active 

 volcanoes. 



It will be seen in the following chapters, that in the earth's crust 

 there are volcanic tuffs of all ages, containing marine shells, which 

 bear witness to eruptions at many successive geological periods. 

 These tuffs, and the associated trappean rocks, must not be compared 

 to lava and scoriae which had cooled in the open air. Their counter- 

 parts must be sought in the products of modern submarine volcanic 

 eruptions. If it be objected that we have no opportunity of studying 

 these last, it may be answered, that subterranean movements have 

 caused, almost everywhere in regions of active volcanoes, great changes 

 in the relative level of land and sea, in times comparatively modern, 

 so as to expose to view the effects of volcanic operations at the bottom 

 of the sea. 



Thus, for example, the examination of the igneous rocks of Sicily, 

 especially those of the Val di Noto, has proved that all the more 

 ordinary varieties of European trap have been there produced under 

 the waters of the sea, at a modern period ; that is to say, since the 

 Mediterranean has been inhabited by a great proportion of the exist- 

 ing species of testacea. 



These igneous rocks of the Val di Noto, and the more ancient 

 trappean rocks of Scotland and other countries, differ from subaerial 

 volcanic formations in being more compact and heavy, and in forming 

 sometimes extensive sheets of matter intercalated between marine 

 strata, and sometimes stratified conglomerates, of which the rounded 

 pebbles are all trap. They differ also in the absence of regular cones 

 and craters, and in the want of conformity of the lava to the lowest 

 levels of existing valleys. 



It is highly probable, however, that insular cones did exist in some 

 parts of the Val di Noto ; and that they were removed by the waves, 

 in the same manner as the cone of Graham Island, in the Mediterra- 

 nean, was swept away in 1831, and that of Nyoe, off Iceland, in 1783.* 

 All that would remain in such cases, after the bed of the sea has been 

 upheaved and laid dry, would be dikes and shapeless masses of igne- 



* See Princ. of Geol., Index, " Graham Island,' 

 canic," &c. 



Nyoe," " Conglomerates, vol- 



