Structure of Granite. 3 



other, or on both sides, tliat is, on the declivities of the moun- 

 tain groups, while they do not appear at all in the highest re- 

 gions ; or, if they do so, are associated at the same time with 

 other mountain-rocks. 



These granite masses often exhibit a remarkable internal 

 structure. They consist either of tabular, or of cubical and 

 cuboidal, or of irregularly-formed masses, so that in the last 

 case they are bounded by an indefinite number of surfaces 

 placed in an entirely indefinite position. These surfaces are 

 by some regarded as rents (kluften), that is, bounding sur- 

 faces which have been produced by external agency after the 

 formation and induration of the rock. When, however, we 

 break a piece of granite bounded by such surfaces, and compare 

 the fractured surface thus produced with these surfaces, we find 

 that the two are of an entirely different description, and there- 

 fore must have had a different origin ; and, when we carefully 

 examine the first, we remarlv that the individual component 

 parts, that is, the individual parts of which granite consists, in 

 so far as they appear in these so-termed rents, are bounded by 

 surfaces which correspond essentially with those by which the 

 individuals of a twin crystal, or by which granular or columnar 

 distinct concretions of calcareous spar, are separated. What 

 were formerly termed abgesonderte stucke (distinct concre- 

 tions) in mineralogy, are now termed zusammensetzungs- 

 stucke (literally, composition-pieces), because the massive 

 portions of minerals, twin crystals, &;c., are composed of the 

 same ; and the surfaces which bound these are termed sm*- 

 faces of distinct concretions ( zusamniensetzungsjidchen). ♦ 

 Besides granite, excellent examples of this structure are 

 afforded by syenite, porphyry, basalt, greywacke (especially 

 the large granular varieties), &c. The indeterminately angu- 

 lar pieces of which a mass of granite is composed, are bounded 

 by surfaces which consist of the distinct-concretion-surfaces 

 of the individual component parts which meet there ; and 



* Ab we are not in possession of such refinements in our geognostical no- 

 menclature, wc must throughout this article, as we did in a former one, 

 vol. 28, p. 334_, continue to employ the expression distinct concretions for 

 ^K^ammmsetzuuf/s-stiickc, and suifaces of distinct concretion for ^Ksammcinet' 

 zwigs-fidcMiu — ^Edit. 



