EXPLANATION OF PLATE 1. 
7 
relations to Transition and Secondary strata ; f. 6. represents 
an example of an extensive eruption of Basaltic matter, 
over Chalk and Tertiary strata, accompanied by an intru- 
sion of vast irregular masses of the same materials into the 
body of the subjacent Primary and Transition rocks. 
f- V. represents strata of columnar Basalt, immediately 
beneath streams of cellular Lava, in regions occupied also 
Ijy craters of extinct Volcanos, f. 8. represents similar 
beds of columnar lava in the vicinity of active V olcanos. 
Trachyte and Lava. 
The fourth and last class of intruded rocks, is that of 
niodern volcanic Porphyries, Trachytes,* and Lavas. The 
undeniable igneous origin of rocks of this class forms the 
strongest ground-work of our arguments, in favour of the 
Igneous formation of the older unstratified and crystalline 
rocks ; and their varied recent products, around the craters 
of active Volcanos, present gradations of structure, and 
composition, which connect them with the most ancient 
Porphyries, Sienites, and Granites. 
The simplest cases of volcanic action are those of Tra- 
chyte (g. ].) and of Lava (i. 5.) ejected through apertures 
in Granite ; such cases prove that the source of volcanic 
fires, is wholly unconnected with the pseudo-volcanic re- 
sults of the combustion of coal, bitumen, or sulphur, in 
stratified formations, and is seated deep beneath the Pri- 
mary rocks.f 
The appellation of Trachyte has been given to a volcanic Por- 
P yry, usually containing Crystals of glassy felspar, and remarkably 
arsh to die tonch, (hence its name from rpax^'t) > d d®®® uot occur in 
ritain, hut abounds in the neighbourhood of almost all extinct and 
active volcanic craters. 
t The occurrence of angular fragments of altered Granite, em- 
edded in Pillars of columnar Lava, in die valley of Monpezat in the 
rdeche, shews that these fragments were probably torn off during 
