10 /. Ruskin — Banded aad Brecciated Concretions. 



the snail, being air-breathing, always takes a reverse position ; and 

 any one who will examine an old wall when snails are hybernating 

 will find them attached most frequently to the roof of a cavity with 

 their shells downwards, sometimes on an upright part distant from 

 the opening in the wall through which they enter ; but I have never 

 seen one on the floor of the cavity, or anywhere where water could 

 accumulate : and it is precisely in accordance with this habit and in 

 such shaded situations that these holes are found, and which are 

 bored upwards for the same reasons. The same holes, therefore, 

 would not be suitable for the aquatic and the air-breathing Molluscs : 

 what would be life to the one would be death to the other. 



In conclusion, the fair inference to be drawn from the foregoing 

 appears to be that the holes and chambers in question are not the 

 work, even partially, of Pholades, or of any other marine Mollusc, 

 and therefore are of no value as a testimony to any movement or 

 elevation of the land. But, all the facts being considered, as well 

 as the distribution of the holes in so many limestone districts where 

 snails abound, at such various altitudes, and so far asunder, the 

 position of the holes and their form agreeing with the habit of the 

 Mollusc, it may reasonably be assumed that Dr. Buckland was 

 correct in the conclusion he drew, in his paper published in 1841, 

 that the holes were made by Helices- — not, however, as M. Bouchard 

 supposed, by the chemical action of the snail secretion alone, but by 

 the mechanical rasping action of its odontophore. assisted, possibly, 

 by an acid secretion from its mouth. 



w 



III. — Banded and Brecciated Concretions. 



By John Rtjskin, LL.D., F.G.S. 



(PLATE II.) 



{Continued from the December (1869) Kumber, page 534.) 



E have now, I think, obtained sufficient evidence that the 

 disposition of differently coloured or composed bands in agate 

 is in most cases the result of crystalline segregation. We shall 

 find, also, that the order of this segregation is constant under given 

 conditions ; and that, with fixed proportions of elements and fixed 

 rate of cooling and drying, the agate wilh necessarily produce itself 

 in a riband of a fixed succession or pattern of stripes : a spectrum 

 of substances, which, if we had observed data enough, we might 

 read like a spectrum of light ; inferring, not the nature of the 

 elements from its bars of colour, but the former conditions of solu- 

 tion from the bars of elements. 



Wlicn the stone has been midisturbed, this riband or chord of its 

 constituent elements will necessarily form qiiietly roimd it, either in 

 its nest, or on its nucleus, with phases of level or v^ertical deposit 

 under peculiar circumstances. But when the congelation has been 

 disturbed, the chord of elements is broken up, and may then be 

 traced here and there about the stone, forming where it may, and as 



