454 Queries and Olservalions relating la 



geologists who exclude the agency of fire, from the format! oh of 

 rocks, seem to forget that the only instances we have of actual 

 rock formations are volcanic : beds and strata more than thirty 

 miles in length, and of considerable breadth and thickness, have 

 been spread over the surface of the globe in our own times: and 

 according to Mr. Humboldt, the further back we trace these erup- 

 tions, the greater is the similarity between the currents of lava 

 and those rocks which are considered by geologists as the most 

 ancient. The enormous volcanoes whose craters are many 

 leagues in extent had doubtless an important office to perform in 

 nature : and can it be unreasonable to believe that the earth it- 

 self is the great laboratory and storehouse where the materials 

 that form its surface were prepared, and from whence they were 

 thrown out upon the surface in an igneous, aqueous, or gaseous 

 state, either as melted lava, or in aqueous solution, or in me- 

 chanical admixture with water in the form of mud, or in the 

 comminuted state of powder or sand ? Inflammable and more 

 volatile substances may have been emitted in a gaseous state, and 

 become concrete on the surface. 



These primaeval eruptions, judging from the size of the an- 

 cient craters, may have been sufficient to cover a large portion 

 of the globe. Nor can it be deemed improbable that still larger 

 and more ancient craters have been entirely covered by succeed- 

 ing eruptions. In proportion as the formation of the surface 

 advanced, these eruptions might decline, and, when their office 

 was performed, might finally cease. 



It is not necessary to suppose that these subterranean erup- 

 tions consisted only of lava in a state of fusion. The largest 

 active, volcanoes at present existing, throw out the different earths 

 intermixed with water in the form of mud. Nor should we li- 

 mit the eruptions of earthy matter in solution or suspension to 

 the known volcanic craters: the vast fissures or rents which in- 

 tersect the different rocks may have served for the passage of 

 the subterranean matter rising to the surface. Silex or quartz, 

 either pure or combined with other earths, constitutes two-thirds 

 of the crust of the globe ; and the veins which intersect the 

 lowest granitic and schistose rocks are most fre(j,uently filled with 

 this mineral. Whether the elementary parts of silex are easily 

 soluble in water, or fusible by fire, we have yet to learn ; but we 

 know there are natural processes by which its solution is effected, 

 and may not the strata of crystalline sandstone have been formed 

 from these solutions ? Calcareous or cretaceous matter is also 

 ejected during aqueous eruptions. (See page 317.) The beds 

 of limestone may have been formed by similar calcareous erup- 

 tions ; and the nunierous remains of shell-fish in Hmestone might 

 pppear to indicate that the calcaveous solutions were favourable 



to 



