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TRANSXTION ROCKS. 



CHAPTER XXI. 

 GEOLOGY OF THE UNITED STATES. 



TRANSITION ROCKS. 



Definition.— Clay Slate.— Its Distribution.— Transition Lime- 

 stone. — Mr. M'CJure's Description of this Formation. — Mr. 

 Featherstonhaugh's Old Red Sandstone. — Its Geological Dis- 

 tribution. 



We have already remarked that the transition 

 rocks are those that rest on the primary, and, as 

 they seem to pass into them, they are called transi- 

 tioriyivom the Latin words trans and eo, to go or pass 

 over. They are sometimes called intermediate ; 

 but as it is difficult to tell where they begin or where 

 they end, some geologists, as Professor Sedgwick, 

 Mr. Brown, &c., merge them either wholly in the 

 primary, or partly in that and partly in the second- 

 ary classes. A s the earliest of the transition class, 

 argillaceous slate, does contain fossil* organic re- 

 mains, we find it necessary, from our definition of 

 primary rocks, and far more suited to the existing 

 order of nature, to place this rock, which has gener- 

 ally been arranged among the letter, among the 

 transition series. But, without entering on the un- 

 profitable dispute whether clay slate should belong 

 to the transition, or whether, indeed, there should 

 be any transition class or not, our object will be at- 



* "This is the oldest system of strata in which organic re- 

 mains are certainly known to occur ; and it may surprise the 

 speculators in cosmogony to hear that these, the most ancient 

 forms of life known to us, should be not plants, but animals 

 &c. — Philips's Geology, vol. i., p. 128. 



