360 TRAP ROCKS. [Ch. XXV. 



rocks composed chiefly of augite, hornblende, and felspar, 

 which are now admitted by all to have been once in a state of 

 fusion, they were divided in opinion whether they were of 

 igneous or of aqueous origin. We have shown in our sketch 

 of the history of geology in the first volume, how much the 

 polemical controversies on this subject retarded the advance- 

 ment of the science, and how slowly the analogy of the rocks in 

 question to the products of burning volcanos was recognized. 



Most of the igneous rocks first investigated in Germany, 

 France, and Scotland, were associated with marine strata, and 

 in some places they occurred in tabular masses or platforms 

 at different heights, so as to form on the sides of some hills a 

 succession of terraces or steps, from which circumstance they 

 were called trap' by Bergman (from trappa, Swedish for a 

 staircase), a name afterwards adopted very generally into the 

 nomenclature of the science. 



When these trappean rocks were compared with lavas pro- 

 duced in the atmosphere, they were found to be in general less 

 porous and more compact ; but in this instance the terms of 

 comparison were imperfect, for a set of rocks, formed almost 

 entirely under water, was contrasted with another which had 

 cooled in the open air. 



Yet the ancient volcanos of Central France were classed, 

 in reference probably to their antiquity, with the trap rocks, 

 although they afford perfect counterparts to existing volcanos, 

 and were evidently formed in the open air. Mont Dor and 

 the Plomb du Cantal, indeed, may differ in many respects from 

 Vesuvius and Etna in the mineral constitution and structure of 

 their lavas; but it is that kind of difference which we must ex- 

 pect to discover when we compare the products of any two active 

 volcanos, such as Teneriffe and Hecla, or Hecla and Cotopaxi. 



The amygdaloidal structure in many of the trap formations 

 proves that they were originally cellular and porous like lava, 

 but the cells have been subsequently filled up with silex, car- 

 bonate of lime, zeolite, and other ingredients which form the no- 

 dules. Dr.Macculloch, after examining with great attention the 



