OVERLYING AND TRAP ROCKS. 



undistinguishable from the cavities of volcanic scoria. 

 And this island must be formed of trap, if the neigh- 

 bouring tracts in Bute and on the Ayrshire coast are ; 

 because they are all, unquestionably, parts of one 

 general deposit. The same argument is indeed equally 

 derived from all the amygdaloids ; under the explana- 

 tion which I formerly gave of the origin of the imbedded 

 nodules. And if this is therefore no such test as has 

 been asserted, it is not true that the trap rocks have 

 been necessarily subjected to great pressure, or, as 

 inferred, that they have all been formed beneath a 

 deep sea. Nor is there even the imagined necessity 

 for this supposition on the part of these superficial 

 system-makers ; since the general -question of the 

 igneous origin of trap is not implicated in the esta- 

 blishment of that v opinion. Many rocks of this nature 

 may have been thus formed, beneath, or among, the 

 strata with which they are now associated above the 

 level of the ocean, having thus also elevated them 

 from beneath the water ; as the volcanoes of Italy and 

 the Coral islands have moved the superincumbent strata 

 to their present positions. But it is also a mere dis- 

 pute about terms, to refuse the name of submarine 

 volcanoes to these eruptions of trap. They are such 

 in every essential point, if they do not now eject fire or 

 smoke ; they are extinguished volcanoes, as much as 

 are those tracts in Italy where similar elevations of the 

 strata have occurred, and where the volcanic action 

 has ceased. And if the limitation of formations of 

 trap to the sea, while the land has escaped them, is 

 absurdly hypothetical, it is equally unnecessary ; since 

 the pressure required to prevent the extrication of air 

 may have taken place on the land as well as under the 

 water ; as is fully proved in Volcanoes, by the existence 

 of solid lavas, which, in other situations, would be 



