EXPLANATION OF PLATE I. 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. 7. represents strata of columnar Basalt, immediately 

 beneath streams of cellular Lava, in regions occupied also 

 by craters of extinct Volcanoes, f. 8. represents similar 

 beds of columnar lava in the vicinity of active Volcanoes. 



Trachyte and Lava. 



The fourth and last class of intruded rocks, is that of 

 modern 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 Volcanoes, 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. 1.) 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 Primary 

 rocks.f 



* The appellation of Trachyte has been given to a volcanic Por- 

 phyry, usually containing Crystals of glassy felspar, and remarkably 

 harsh to the touch, (hence its name from Tga^vc); it does not occur in 

 Britain, but abounds in the neighbourhood of almost all extinct and 

 active volcanic craters. 



f The occurrence of angular fragments of altered Granite, em- 

 bedded in Pillars of columnar Lava, in the valley of Monpezat in the 

 Ardeche, shows that these fragments were probably torn off during 



