24 Principles of Geology. 



condition of nature, such soils suit such trees, these circumstances 

 have led to an opinion that the trees grew near the spots where they 

 lie buried.. If this be thought sufficiently probable, we arrive at a 

 startling conclusion ; for as these trees are often buried some yards 

 below the usual level of the sea, and are sometimes, as on the shore 

 of South Wales, covered thirty feet deep by the tide, it would ap- 

 pear that the sea has risen so much on our coasts. If the levels of 

 Yorkshire were once covered with forests of oak, the sea must have 

 been debarred access to them, and it would seem, therefore, that its 

 general level has been since much raised ; for those trees 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, produ- 

 ced by sudden eruptions of volcanoes, are less considerable than 

 those occasioned by the slow and continued action of water. It is 

 along mountain 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 exten- 

 sive 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 admissible, 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 

 geologists, 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 indepen- 

 dent 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 phenomena appear, which strongly confirm 

 this reasoning. Thus, where basalt passes through coal, this mineral 

 has lost its bituminous portion, as efliectually as if it had undergone 

 distillation. Ordinary limestone, divided by basalt, exhibits a crys- 

 tallized 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 independent series of rocks derive 



