CLASSIFICATION OF PRIMARY ROCKS. 



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primary rocks, in immense masses below all other rocks, have in 

 some situations, reproduced them in smaller masses, covering rocks 

 belonging to the transition or secondary classes. 



Granite, for instance, which has been regarded as the most an- 

 cient of all known rocks, has been sometimes found covering secon- 

 dary rocks, and sometimes obtruded between them. Facts of this 

 kind are rare, and can be explained only by admitting that granite, 

 like volcanic rocks, has once been in a state of fusion, and was pro- 

 truded in this state through the upper rocks. Similar facts are ob- 

 served with respect to other primary rocks, which are believed to be 

 of igneous formation. 



Indeed, if the science were sufficiently advanced to enable us to 

 pronounce, with absolute certainty, on the agents by which rocks 

 were formed, a more intelligible arrangement might be substituted^ 

 than one founded on their relative ages 5 it might be comprised in 

 three great divisions. 



Class I. Rocks of igneous Formation. 

 Class II. Rocks of aqueous Formation. 

 Class III. Conglomerates, and mechanical Formations. 



These would admit of distinct subdivisions : — 



Class I. a. Rocks that have been fused and consolidated with- 

 out ever having flowed as lavas. 

 h. Rocks that have been fused and protruded through 



the solid covering of the globe, 

 c. Rocks that have been greatly modified by heat, but 

 which were originally aqueous depositions. 

 Class II. a. Marine formations. 



b. Freshwater formations. 

 Class III. a. Ancient conglomerates. 



b. Recent conglomerates. 

 Each of these divisions would comprise rocks of different relative 

 ages ; that of rocks of the first class would be determined by their 

 position ; those of the second and third classes by their order of suc- 

 cession, and the organic remains in each.'^ 



Such an arrangement might be objected to, as resting too much on theory , 

 and the fate of the Wernerian system ought to caution ns against founding sys- 

 tems of classification on theoretical views respecting the formation of rocks. The 

 following rocks, according to the evidence at present obtained, might be referred 

 to the different divisions of the first class ; and it may be useful to bear this in 

 mind, without yielding implicit assent to the theory that they are all igneous for- 

 mations; yet it must be allowed, that such a mode of formation will satisfactorily 

 account for many positions in which these rocks occur, that appear inexplicable 

 by any other theory. 



Supposed igneous Rocks. — All varieties of granite, gneiss, and mica slate ; — all 

 varieties of porphyry and felspar rocks; — all varieties of hornblende rocks and 

 serpentine ;— all basaltic or trap rocks ; — all ancient and recent lavas. 



In subdivision c, slate rocks, clay-slate, and crystalline limestone, imbedded in 

 igneous rocks. 



