Ruskin — On Banded and Brecciated Concretions, 157 



chalcedony, and the whole enclosed by a quartzose crystalline mantle 

 or crust. 



The British Museum specimen {a, h, d, 5) is said to be Icelandic. 

 Two others of the group are labelled " Oberstein ; " one, of parallel 

 construction, but slightly varied in character, is from Zweibrucken, 

 in Bavaria. I do not know the locality of my own, but there is a 

 community of feature in all the specimens, which assuredly indicates 

 similarity of circumstance in their localities ; and the more various 

 the localities, the more interesting it will be eventually to determine 

 their points of resemblance. I have not yet obtained an example 

 of this group in the gangue, but the crust of the stones themselves 

 is in every case composed of quartz-crystals rudely formed, some- 

 times so minute as to look like a crumbling sandstone : in my own 

 specimen they can only be seen with a lens, associated in filiform 

 concretions like moss ; within this crust two distinct formations have 

 first taken place, and then a change of state is traceable affecting 

 both in new directions. The map-diagram, PL X. Fig. 2, is lettered, 

 so as to permit accurate indication of the parts. ^ 



The outer formation, next the crust, is composed of very pale 

 whitish brown jasper. It is expressed by a shade of grey in the 

 map, and is limited towards the interior of the stone by the strong 

 line (with occasional projecting knots) thrown into curves, convex 

 outwards. 



The inner formation is of a finer jasper, with dark chalcedony in 

 segregation. The vertical lines in the map indicate the chalcedony, 

 and the pure white space, the inner jasper, terminated outwardly by 

 convex curves. We are thus led at once to note the distinction 

 between the two families of agates, formed from within outwards in 

 knots, and from without inwards in nests. The first group, to which 

 our present example belongs, is usually agatescent in the interior, 

 and crystallized on the surface; the second is agatescent in the 

 coating, and crystallized in the interior. 



Supposing the silica deposited under the same circumstances of 

 solution, and the same time granted for solidification, the difference 

 between these two structures would depend (and often does de- 

 pend) only on the chance of the silica finding a hollow prepared 

 for its reception, or a solid nucleus round which it can congeal; 

 the ordinary deposits on the inner surface of a nest often become 

 nodular or stalactitio as they project into its open space, and the 

 greater part of the apparently independent concretions are probably 

 mere fragments out of the hollows of larger ones. Bat there is, 

 nevertheless, frequently a true distinction between the two modes of 

 deposit. The agates formed on a central nucleus appear usually to 

 have had a longer time for their construction than those which fill 

 hollows, or, at least, they are the portion of the mass, in the hollow 

 itself, which has crystallized most slowly ; they are distinctly reni- 

 form in their chalcedony, and distinctly symmetrical in their crystals ; 



^ 1 have carelessly worded the title of Plate X. as if the two figures were a vertical , 

 section and surface map ; but the lower one is, of couise, only explanatory of the 

 upper. 



