208 Ruskin — Banded and Brecciated Concretions. 



The inspection of this table will shew that the known rising and 

 sinking of large areas of the earth's surface is adequate to produce 

 much compression and extensive faults. But this table can be made 

 to yield some other important results. 



When the geological structure of a country is pretty well known, 

 the amount of contortion, — i.e. the difference between the direct dis- 

 tances of two distant points, (1) measured along a circular arc, (2) 

 measured along strata, — may become approximately known. And if 

 this contortion appears not to be due to the intrusion of local 

 igneous rocks, and does appear to be due to depression, we get a 

 means of calculating to what depth the strata sank when those con- 

 tortions were being produced. 



And, again, it appears that the horizontal projection of the fault, 

 or, in other words, the '' barren ground," is the measure of the 'power 

 of a fault. Hence, if the vertical throw of a fault be /, and the in- 

 clination of its plane to the vertical be 6, and the barren ground be 

 therefore / tan ^, it appears that X (/ tan^), taken along any 

 section, is a measure of the subsequent elevation. But X (/ tan 6) 

 is an observable quantity, independent of hypotheses : hence we 

 may be able to infer the amoimt of re-elevation, which ought to cor- 

 respond approximately with the amount of depression obtained for 

 the same section from the contortions. 



But apart from this application of my explanation of the origin 

 of contortions and faults, which is not yet, I imagine, practicable, 

 I should be glad to know the opinion of geologists about the ex- 

 planation itself. 



III.-— On Banded and Brecciated Concretions. 



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



(PLATE XIII.) 



(Continued from the April Number, p. 161.) 



THE next group of agates which I have to describe belongs to the 

 nested series ; but is distinguished from all other varieties of 

 that series by having a pure chalcedonic surface (unaffected, except 

 in the form of it, by the material of its gangue) ; and by uniformity 

 of colour ; consisting only of white and transparent grey bands, 

 wholly untinged by more splendid colours. But nearly all the agates 

 of this group which now occur in the market have been dyed brown 

 or black at Oberstein, to the complete destruction of their loveliest 

 phenomena. 



With the true agates of this group must be associated some 

 transitional examples, in which the surface is more or less entangled 

 with, and degraded by, the material of the gangue, (the body of the 

 stone then becoming susceptible of colouring by iron, or of chloritic 

 arborescence from the exterior) ; and others, in which the mass is 

 rudely egg-shaped, like a rolled pebble, and the crust is of a fine 



