126 ANCIENT AND MODEBN IGNEOUS BOCKS. 



state. The latter hypothesis, proposed by Sir Humphry 

 Davy, soon after his discovery of the metallic bases of the 

 earths, was subsequently abandoned by him, but has since 

 been revived by Dr. Daubeny. 



SIMILARITY OF THE ANCIENT AND MODEEN IGNEOUS 

 HOCKS. In taking a general review of the plutonic and the 

 volcanic rocks, it is impossible not to perceive such points of 

 similarity as serve to demonstrate their common origin, and 

 prove them all to be vast portions of melted matter which 

 have been poured out at various times, and under different 

 circumstances, from the interior of the earth. "We have 

 alluded to the similarity existing between the products of 

 modern volcanoes and the ancient plutonic rocks, as affording 

 numerous and valid proofs of the igneous nature of both, 

 the student may select the following, as the most convincing 

 of these. 



1. They both bear, in many instances, a close resemblance 

 in mineral composition; many of the ancient basalts, as already 

 stated, being scarcely distinguishable from the productions 

 of existing volcanoes. 



2. They alike contain mineral substances and crystals in 

 such abundance, as to constitute the chief source whence 

 these objects are derived. 



3. They graduate into rocks of like nature and origin with 

 themselves ; thus granite passes through syenite and green- 

 stone into basalt, and this last to pitchstone, in a manner 

 analogous to the transition of modern trachyte and basalt 

 into obsidian. 



4. They intrude into other rocks, and calcine and alter 

 them at the point of contact, converting coal into coke and 

 anthracite ; shale, into mica-schist, or flinty slate ; and beds 

 of sandstone into quartz-rock, in so striking a manner that 

 it is possible to trace the effects from the very point of meet- 

 ing, and to note the passage from the altered to the unaltered 

 rock. The same result, under varied conditions, is also 

 recognised in modern volcanic productions. 



To enumerate the whole of the facts which might be 

 adduced to prove the identity of the plutonic and volcanic 

 rocks, would rather burden than assist the memory; and 

 the few proofs above-mentioned, with others which will 

 suggest themselves, will amply suffice. 



