Ch. XII.] NATURE OF STRATIFICATION. 253 



have observed appearances in granite, trap, porphyry, and 

 similar rocks, which resemble those commonly attributed 

 to stratification, have explained the apparent anomaly by 

 assigning it to the concretionary structure of these rocks. 

 Dr. Macculloch, in particular, has written much on this 

 subject, a digest of which has been published in a connected 

 form in his System of Geology * : and as his view of the 

 subject has not been objected to, it may be therefore pre- 

 sumed that it is generally received. We must confess that it 

 appears to us more specious than correct ; and as a proof that 

 the proposed discrimination is not a faithful sketch of nature, 

 the student cannot, however diligent he be, reduce this 

 precept into practice. The cause of this, if our deduction 

 be not erroneous, is very evident; a distinction has been 

 attempted, where none really exists : for all perfectly con- 

 solidated rocks possess a concretionary structure. It matters 

 not whether the rock has had an igneous, aqueous, or any 

 unknown origin ; its particles were once disunited or mobile, 

 either by the repulsive agency of caloric, by chemical solution, 

 or suspension in water, by mechanical attrition, or by some 

 other cause ; subsequently, however, its particles have been 

 brought within the sphere of their mutual attraction, by a 

 reduction of temperature, by precipitation, by great pressure, 

 by the percolation of water imparting the requisite degree of 

 motion, or introducing extraneous matter, by which the sepa- 

 ration of the particles is diminished or cemented; or, lastly, 

 by any two or more of these compatible causes, which can 

 operate in unison to effect the cohesion of the particles. We 

 need not, therefore, enquire what has been the origin of a 

 rock, before we determine whether it has a concretionary 

 structure or not : since, in a mass of lava, of travertin, or of 

 recent sandstone, each composed of lesser masses or concre- 

 tions, more or less symmetrical, we learn that their structure 

 is not exclusively dependent on any one particular mode or 

 formation, for each individually, by placing the particles of 



* Vol. i. p. 166. 



