THE 



GEOLOGICAL MAGAZINE. 



No. XLI.— NOVEMBER, 1867. 



I. — On Brecciated Formations. 



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



(PLATE XX.) 



(Continued from the August Number, p. 339). 



IWEOTE the first of these papers more with a view of obtaining 

 some help in my own work than with any purpose of carrying 

 forward the discussion of the subject myself. But no help having 

 been given me, I must proceed cautiously alone, and arrange the 

 order of my questions ; since, when I have done my best as care- 

 iullj as I can, the papers will be nothing but a series of suggestions 

 for others to pursue at their pleasure. 



Let me first give the sense in which I use some necessary words : 



1. Supposing cavities in rocks are produced by any accident, or 

 by original structure (as hollows left by gas in lava), and after- 

 wards filled by the slow introduction of a substance which forms an 

 element of the rock in which the cavities are formed, and is finally 

 present, in the cavities, in proportion to its greater or less abundance 

 in the rock ; I call the process " secretion." 



2. But if the cavities are filled with a substance not present (or 

 not in sufficient quantity present), in the surrounding rock, and 

 therefore necessarily brought into them from a distance, I call the 

 process, if slow, "infiltration": if violent, "injection." 



It is evident that water percolating a rock may carry a substance, 

 present in the mass of it, by infiltration, into the cavities, and so 

 imitate the process of secretion. But there are structural differences 

 in the aspect of the two conditions hereafter to be noticed. The ex- 

 istence of permanent moisture is however to be admitted among 

 conditions of secretion; but not of fluent moisture, introducing 

 foreign elements. 



3. If a crystalline or agatescent mass is formed by addition of 

 successive coats, I call the process " accretion." 



4. But if the crystalline or agatescent mass separates itself out of 

 another solid mass, as an imbedded crystal, or nodule, and then, 

 within its substance, divides itself into coats, I call the process 

 " concretion." The orbicular granite of Elba is the simplest instance I 

 can refer to of such manifest action ; but all crystals, scattered 

 equally through a solid enclosing paste, I shall call " concrete " 

 crystals, as opposed to those which are constructed in freedom out of 



vol. IV. — NO. XLI. 31 



