II^TIIOIDUOTION. 



The most ancient rocks known to man had their immediate origin in sedi- 

 mentary deposition at the bottom of an ocean. At this point commences Ge- 

 ology ; for previous to this period nothing has been ascertained as to the condition 

 of the Earth. It commences at the base of the lowest rocks discovered, and 

 from thence it investigates the overlying strata, the changes that have taken 

 place, the lapse of time, and the development of animal life, down to the present 

 moment. 



Were it possible to obtain a transverse section of these rocks at their maxi- 

 mum tliickness, at any particular locality, there would be presented to the view 

 an exposure of more than thirty miles, representing the geological column, which 

 is regarded as one continuous whole, from the base to the top. 



Geologists have adopted various divisions and subdivisions of this column, 

 for the purpose of illustrating the different aspects of the science. But that 

 which subdivides it into groups seems to be the most natural and systematic, 

 and is the classification now generally preferred. 



In the infancy of geological science, the known strata were divided into for- 

 mations, each of which was supposed to represent a geological period, during 

 which no marked changes occurred in the condition of the Earth or in its or- 

 ganic life. The commencement of each formation was supposed to mark a new 

 epoch of creation, and its close to represent a great cataclysm that destroyed all 

 vegetable and animal organisms. Modern geologists, however, have determined 

 that the evidence of special creations and cataclysms is entirely wanting, and 

 that the formations do not mark geological periods of repose, nor are they any 

 more distinct in their character, from each other, than are the groups into which 

 they have been subdivided. 



The palaeozoic rocks, as understood by geologists, embracfe the Permian 

 group, and all that is below it. The subdivision into Archaean, Silurian, Devo- 

 nian and Carboniferous formations is retained as a matter of convenience, for the 

 purpose of directing attention, in a general way, to a particular quarter of these 

 rocks. The Archaean (most ancient) are the metamorphic rocks. The Silurian 

 is next in order. The Devonian, from Devonshire, England, is a much smaller 

 formation, and is characterized by its fish remains. The Carboniferous forma- 

 tion, so-called from the fact that it contains the great coal deposits of Europe, 

 concludes the series of palaeozoic rocks. Other formations contain coal deposits, 

 but they are otherwise so distinct, that they are never in danger of being classed 

 with this formation as originally defined. 



