PEAGMENTS OF OTHER EOCKS CONTAINED IN GEANITE. 3 



many places are usually only the interior, harder nuclei of former 

 larger blocks of concentric structure, which remain behind as such 

 after their outer bark, as it wore, has peeled off. Within such 

 round blocks there is frequently an inner concretion, from i to 2 

 feet and more in diameter, which consists essentially of aggrega- 

 tions of mica, and probably may in most cases have been the first 

 inducement to the concentric scaled structure. Locally these con- 

 cretionary enclosures are called 'souls.' This segregation occurs 

 in most varieties of granite, but especially in the finer-grained 

 ones " *. 



A still more recent writer on the geology of Bohemia makes the 

 following observations on the granites of that country : — " One of 

 the most remarkable peculiarities of this variety of granite (horn- 

 blende granite) is the innumerable fine-grained enclosures which 

 it contains. They are always sharply segregated from the con- 

 taining rock, and are of all sizes, from that of the fist to that of a 

 pin's head. The ground-mass of these fragments is of a dark colour, 

 consisting of mica and small quartz-grains, in which white orthoclase 

 crystals are for the most part porphyritically developed. The en- 

 closures also contain hornblende crystals, although not in such 

 quantity as in the enclosing rock. It is difiicult to say any thing as 

 to the mode of production of these fragments ; but from the identity 

 of the constituents and from their great abundance, it would appear 

 that they were not derived from a rock broken through, but the 

 product, during the solidification of the whole mass, of a process of 

 segregation, the nature of which is entirely unknown to us. The 

 circumstance must be noted, however, that they are exclusively 

 confined to the area of the hornblende granite, but entirely absent 

 in the other varieties "f. 



A very remarkable example of concretionary granite is described 

 in the U. S. Report on the Greology of Vermont J. " The basis of 

 this remarkable variety of granite is rather fine-grained, white and 

 highly felspathic. The mica, however, is usually dark, and where 

 it exists in large quantities it gives the rock the aspect of syenite. 

 But there is no hornblende present. Scattered through this base 

 occur numerous spheroidal or elongated and somewhat flattened 

 nodules of black mica, from half an inch to two inches in diameter ; 

 and when elongated the longer axis is sometimes seen as much as 

 four or five inches long. They are usually more or less flattened, 

 and have a shrivelled appearance like dried fruit. They sometimes 



become so thin as to consist only of a few plates When 



the nodule is elongated and the wrinkles correspond, as they always 

 do, to the longer axis, the resemblance is very striking to a dried 



* Johann Jokely, " Geogn. Yerhaltnisse in einem Theile des mittleren 

 Bohmen," Jahrb. k.-k. geol. Reichsanstalt, 1855, p. 375. 



t F. von Andrian, " Beitrage zur Geologie des Kaurimer und Taborer Kreises 

 in Bohmen," ibid. 1863, p. 166. 



\ Report on the Geology of Vermont, by Edward Hitchcock, LL.D., Edward 

 Hitchcock, Jan., M.D., Albert Hager, A.M., and Chas. H. Hitchcock, A.M., 

 vol. ii. p. 564, 1861. 



B 2 



