OP PETRIFACTIONS. 219 



NOTE M. 23. 

 On tht Distribution of Petrifactions in the different 



Classes of Roc/$s 



As an account of the distribution of fossil organic re- 

 mains throughout the strata, of which the crust of the 

 earth is composed, cannot fail to prove interesting, even 

 to the general reader, we shall here give a very short 

 sketch of what is known on the subject. Fossil organic 

 remains, or petrifactions, have not hitherto been disco- 

 vered in any of the primitive rocks ; indeed it would ap- 

 pear that animals and vegetables were not called into ex- 

 istence until the period when the transition rocks began 

 to be formed. Hence it is, that petrifactions have not 

 been met with in any rock older than those of the transi- 

 tion class. 



TRANSITION ROCKS. 



The principal transition rocks are greywacke, grey- 

 wacke slate, clay slate, limestone, greenstone, amygda- 

 loid, syenite, porphyry, and granite. All of them do not 

 afford petrifactions, these bodies having been hitherto 

 found only in limestone, greywacke, greywacke slate, and 

 clay slate. 



1 . Transition Limestone. 



Fossil corallitic bodies, such as madreporites, tubipo- 

 rites, and milleporites, of different species, abound in 

 many varieties of this limestone. It is in general difficult 

 to determine the species of these genera, owing to their 

 being much intermixed with each other, and with the 

 matter of limestone. On a general view, they certainly 

 approach in external characters to those corals we at 

 present meet within a living state in the tropical regions of 

 the globe. Intermixed with these corals, or in separate 

 strata, we find various species of orthoceratites, lituites, 



