THE 



GEOLOGICAL MAGAZINE. 



No. XXXVIII.— AUGUST, 1867. 



I. — On Banded and Bkecciated Concretions. 

 By John Ruskin, Esq., F.G.S., 



(PLATE XV.) 



AMONG the metamorphic phenomena which seem to me deserving 

 of more attention than they have yet received, I have been 

 especially interested by those existing in the brecciate formations. 

 They are, of course, in the main, two-fold ; namely, the changes of 

 fragmentary or rolled-pebble deposits into solid rocks, and of solid 

 rocks, vice versa, into brecciate or gravel-like conditions. It is cer- 

 tainly diificult, in some cases, to discern by which of these processes 

 a given breccia has been produced ; and it is difficult, in many cases, 

 to explain how certain conditions of breccia can have been produced 

 either way. Even the pudding-stones of simplest aspect (as the 

 common Molasse-nagelfluhe of north Switzerland) present most 

 singular conditions of cleavage and secretion, under metamorphic 

 action; the more altered transitional breccias, such as those of 

 Valorsine, conceal their modes of change in a deep obscurity : but 

 the greatest mystery of all attaches to the alterations of massive 

 limestone which have produced the brecciated, or apparently brec- 

 ciated, marbles : and to the parallel changes, on a smaller scale, 

 exhibited by brecciated agate and flint. 



The transformations of solid into fragmentary rocks may, in the 

 main, be arranged under the five following heads : — 



1. Division into fragments by contraction or expansion, and filling 

 of the intervals with a secreted, injected, or infused paste, the degree 

 of change in the relative position of the fragments depending both 

 on their own rate and degree of division, and on the manner of the 

 introduction of the cement. 



2. Division into fragments by violence, with subsequent injection 

 or secretion of cement. The walls of most veins supply notable 

 instances of such action, modified by the influence of pure contraction 

 or expansion. 



8, Homogeneous segregation, as in oolite and pisolite. 



4. Segregation of distinct substances from a homogeneous paste, as 

 of chert out of calcareous beds. My impression is that many so- 

 called siliceous " breccias " are segregations of knotted silex from a 

 semi-siliceous paste ; and many so-called brecciated marbles ai-e segre- 

 gations of proportioned mixtures of iron, alumina, and lime, from an 

 impure calcareous paste. 



VOL. IV. NO. XXXVJII. 22 



