78 NATURE OF SUBTERRANEAN IGNEOUS ROCKS. [Ch. XXVII. 



■ • During tlie last century, about 50 eruptions are recorded 

 of the five EuroiDean volcanic districts, of Yesuvius, Etna, 

 Volcano, Santorin, and Iceland ; but many beneath the sea 



may 



ss 

 a. 



have passed unnoticed. If some of them produced no lav 

 others, on the contrary, like that of the Skaptar Jokul, in 



1 



matter 



which cases, being reckoned as single eruptions, will com- 

 pensate for those of inferior strength. Now, if we consider 

 the active volcanos of Europe to constitute about a fortieth 

 part of those already known on the globe, and calculate that, 

 one with another^ they are about equal in activity to the 

 burning mountains in other districts, we may then compute 

 that there happen on the earth about 2,000 eruptions in the 

 course of a century, or about 20 every year. 



However inconsiderable, therefore, may be the superficial 

 rocks which the operations of fire produce on the surface, we 

 must suppose the subterranean changes now constantly in 

 progress to be on the grandest scale. The loftiest volcanic 

 cones and the lavas which have flowed from their cratei 



^s 



must be insignificant when contrasted with the products of 



fire in the nether regions. 



In regard to these last or those 



ip^neous rocks which have been formed in our o\vn times in 

 the bowels of the earth, whether in rents and caverns, or by 

 the cooling of lakes of melted lava, we may safely infer that 

 they are heavier and less porous than ordinary lavas, and 

 more crystalline, although composed of the same mineral 

 ingredients. As the hardest crystals produced artificially 

 in the laboratory require the longest time for their formation, 

 so we must suppose that where the cooling down of melted 

 matter takes place by insensible degrees, in the course of 



ages, a A^ariety of minerals will be produced far harder than 

 any formed by natural processes within the short period of 

 human observation. 



These subterranean volcanic 



mor 



stratified in the same manner as sedimentary deposits from 

 water, although it is evident that when great masses con- 

 solidate from a state of fusion, they may separate into 

 natural divisions ; for this is seen to be the case in many 



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