THE PRE-ADAMITE EARTH. 



have been enumerated and connected with indications of as many 

 geological convulsions, the result of each of which was the complete 

 destruction of animal and vegetable life upon the globe, the fossil 

 deposits of the next strata of the series in ascending, being the 

 remains of the animal and vegetable tribes which re-peopled and 

 re-clothed the earth after the ensuing period of tranquillity. 



227. The organic femains, deposited in the successive layers of 

 the terrestrial crust, show the character of the animal and vegetable 

 world during the periods of tranquillity which followed the several 

 convulsions which thus devastated the globe. It is, however, 

 a problem of not less high interest to determine the geographical 

 character of the surface, as denned by the relative extent and 

 outlines of land and water, during each of these periods. The 

 solution of this problem would be attended with no difficulty, if 

 we possessed a perfect knowledge of the condition of the sedi- 

 mentary strata in all parts of the earth. But not to mention our 

 want of all knowledge of the state of that large portion of the 

 earth's surface, which is at present covered with water, we have 

 as yet been able to effect only a very limited and imperfect survey 

 even of that lesser part which forms the land. Nevertheless, the 

 positive knowledge, small as it is, which the labours and re- 

 searches of geologists have supplied, aided by obvious analogies 

 on which conclusions are based, having, if not in all cases moral 

 certainty, at least a high degree of probability, has afforded a 

 close approximation to the series of geographical changes which 

 that part of the earth, at present composing the land, has under- 

 gone since the earliest geological epochs. 



228. The means whereby the outlines of land and water at any 

 proposed geological period are determined are twofold, one method 

 depending on the mineralogical character of the strata, and the 

 other upon that of its organic deposits. 



To explain these methods, let us suppose, for example, that it 

 is required to determine the outlines of land and water, upon those 

 parts of the globe of which we possess sufficient geological know- 

 ledge, during the Silurian epoch. The first point will be, in that 

 case, to determine the extent and limits of that class of strata to 

 which the name Silurian has been given. Let us imagine that all 

 the strata of more modern date arc removed, and the Silurian 

 strata laid bare. These strata were deposited during the Silurian 

 period at the bottom of the seas and oceans which then partially 

 covered the globe, and whose outlines and limits are the imme- 

 diate subject of inquiry. The land during this period consisted 

 of those parts of the globe alone upon which the Silurian deposit 

 was not made. If, therefore, we possess the means of discovering 

 the exact extent and outline of the Silurian deposit, we possess 

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