Introduction to Geology. 



67 



of som^ disturbing force which was not extended into the 

 upper series. The products of the lower section are by far 

 the most vahiable in the entire system of rocks. 



There appears considerable difficulty in admitting the sepa- 

 ration of the transition and secondary orders at the point 

 assigned by some writers ; that is, between the mountain lime- 

 stone and the coal measures. The former conforms so com- 

 pletely to the disposition of the beds of the latter, that it really 

 seems impossible to view them but as parts of one great clas^. 

 Neither is the argument founded on the " truly remarkable 

 difference in the nature of the organic remains," in the coal and 

 limestone formations, wholly free from objection ; because the 

 genus Producta is common to the mountain limestone, the 

 shale, and the argillaceous iron ore between the coal seams, in 

 frequent cases, particularly in the great South Wales basin. 



The parallelism, inclination, and curvature of the beds of 

 old red sandstone, are so conformable, also, to those of most 

 coal fields, that it seems equally difficult to separate them there. 

 (See figs. 107. 114. and 1 1 7. Vol. I.) On the other hand, it is 

 urged that the mountain limestone is allied to, if not identical 

 with, the acknowledged transition limestone, and that the " old 

 red sandstone possesses all the mineral characters of greywacke 

 except the colour ; " and Mr. Coneybeare admits that, in 

 many instances, " the limits between this series and that of 

 transition rocks, can only be arbitrarily assigned ; " and fur- 

 ther, that " at least ten characters will be found in common 

 between them, for one which would lead to an opposite ar- 

 rangement." 



So many circumstances, geographical as well as geological, 

 concur to associate these formations, that it appears desirable 

 so to treat them ; and it then remains to be determined whe- 

 ther such associated formations should be transferred to the 

 transition class, or remain the lowest section of the secondary. 

 In the unsettled state of the controversy, we incline to the 

 latter ; and conceive that we should more effectually extricate 

 ourselves from theory, by adopting the alternative offered in 

 Mr. Coneybeare's arrangement, under the characteristic appel- 

 lation of the carhoniferous order. 



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