PICTURESQUE SKETCHES 



In the first place, then, there is the evidence of what 

 are called ' Geological sections;' offering sufficient proof 

 that the different strata of which the earth's crust is 

 composed are of certain limited extent and thickness, 

 and that they overlie one another in regular order. 



Fig. I 



This kind of evidence reaches indeed farther, and 

 proves very sufficiently that there is some character- 

 istic mark of each group of strata by which it 

 may be known and recognised ; so that the kind of 

 sandstone, limestone, or clay beds that may exist in 

 one part of the series, and the order in which they 

 succeed each other, is not so closely imitated in an- 

 other part, but that a distinction may generally be 

 drawn without much difficulty. Geological sections, 

 and the maps which should accompany them, prove 

 also in addition to this, that over large tracts of coun- 

 try, and even over whole continents, the same invari- 

 able order of arrangement of the strata may be traced ; 

 so that the Geologist is thus enabled to advance with 

 some confidence, and frame those generalizations with- 

 out which Geology could hardly exist as a science. 



Besides this evidence, derived from the examination 

 of the mere mineral materials of which strata are com- 

 posed, there is however another, and a far more im- 

 portant means of acquiring a knowledge of the earth's 

 history, derived from the study of the animal and vege- 

 table remains that are found in almost every one of the 



