360 
TRAP ROCKS. 
[Cb. 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 Plombdu 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 
