PRINCIPLES OF GEOLOGY. 2J 



are below the present height of the tide. Alluvial sediments near the 

 sea, on the banks of rivers, and on the site of ancient lakes, enclose 

 shells such as now live in our fresh waters, and bones of the stag and 

 the ox. 



VOLCANOES. The alterations in the features of the globe, produced 

 by sudden eruptions of volcanoes, are less considerable than those oc- 

 casioned by the slow and continued action of water. It is along moun- 

 tain chains, and among mountain groups, that the melted rocks of the 

 interior of the earth are poured forth upon its surface. How deep is the 

 seat of volcanic fire, what is the chemical history of its origin and 

 support, are subjects of philosophical inquiry too extensive to be here 

 discussed. Perhaps the simplest explanation is that suggested by Sir 

 H. Davy's splendid discovery of the metallic bases of the alkalies and 

 earths ; for if to such metals, deep in the earth, water be supposed ad- 

 missible, the combustion which would be so occasioned may be thought 

 equal to produce the phenomena which we behold. 



The most important considerations, which volcanoes suggest to geolo- 

 gists, relate to the substances which they emit ; for some of these fused 

 substances assume, when cold, the appearance of well-known rocks. 

 Some "lavas" closely resemble basalt, others are like pitchstone, and 

 others almost identical with porphyry. Now, these are among the 

 most characteristic of the rocks called independent or overlying ; and it 

 therefore, appears probable, a priori, that such rocks are of igneous 

 origin. On examining the circumstances which accompany them, we 

 find that, where they are in contact with other rocks, particular phe- 

 nomena appear, which strongly confirm this reasoning. Thus, where 

 basalt passes through coal, this mineral has lost its bituminous por- 

 tion, as effectually as if it had undergone distillation. Ordinary lime- 

 stone, divided by basalt, exhibits a crystallized texture, such as Sir J. 

 Hall produced in it by great heat and pressure combined. From an 

 extensive series of such facts, it is inferred that the overlying or indepen- 

 dent series of rocks derive their peculiar appearances, and have produced 

 the remarkable phenomena which accompany them, from the agency of 



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