116 LYELL'S ELEMENTS OF GEOLOGY. 



Relation of Trap, Lava, and Scoriae. 



that is to say, since the Mediterranean has been inhabited by a 

 great proportion of the existing species of testacea. 



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

 trappean rocks of Scotland and other countries, differ from sub- 

 aerial 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 con- 

 formity 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 Mediterranean, 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 igneous rock, cut- 

 ting through sheets of lava, which may have spread over the 

 level bottom of the sea, and strata of tuff, formed of materials 

 first scattered far and wide by the winds and waves, and then 

 deposited. Trap conglomerates also, to which the action of the 

 waves must give rise during the denudation of such volcanic 

 islands, will emerge from the deep whenever the bottom of the 

 sea becomes land.* 



The proportion of volcanic matter which is originally subma- 

 rine must always be very great, as those volcanic vents which 

 are not entirely beneath the sea, are almost all of them in 

 islands, or, if on continents, near the shore. This may explain 

 why extended sheets of trap so often occur, instead of narrow 

 threads, like lava streams. For, a multitude of causes tend, near 

 the land, to reduce the bottom of the sea to a nearly uniform 

 level, the sediment of rivers, materials transported by the 

 waves and currents of the sea from wasting cliffs, showers of 

 sand and scoriae ejected by volcanos, and scattered by the wind 

 and waves. When, therefore, lava is poured out on such a sur- 

 face, it will spread far and wide in every direction in a liquid 

 sheet, which may afterwards, when raised up, form the tabular 

 capping of the land. 



As to the absence of porosity in the trappean formations, the 

 appearances are in a great degree deceptive, for all amygdaloids 



* See Principlesof Geology, Index, " Graham Island," " Nyb'e," " Conglome- 

 rates, volcanic." &c. 



