AGATES AND AGATE-WORKING. 
27 
But though there may not be much difficulty is tracing the 
origin of the silica, there are on the contrary extreme difficulties 
in seeking to interpret some of the appearances presented by 
agates. Why, for example, should the silica, in one and the 
same stone, be sometimes deposited in the form of chalcedony, 
and sometimes shot forth as crystallized quartz ; now deeply 
coloured as bright red jasper, and now delicately tinted as 
purple amethyst ; at one time affecting a crystalline condition, 
and at another time colloidal ? Such alternations in the character 
of the deposits must have recurred again and again in the his- 
tory of many banded agates. The successive strata differ con- 
siderably in texture, hardness, transparency, colour, and other 
physical properties ; but what has determined these differences ? 
Layer after layer has been spread in equal thickness over all the 
irregularities of surface, each coat exquisitely thin and delicate ; 
Sir D. Brewster measured the thickness of some of these strata, 
and found them between lT * 2 0 - and 5-5-yg-o of an inch.* How 
have layers of such extreme tenuity, and yet continuous, been de- 
posited all round the inner walls of an irregularly shaped cavity ? 
These are questions which, simple as they may seem to some at 
the first blush, will be found to grow in difficulty the more care- 
fully they are studied. 
Jakob Noggerath, the venerable professor at Bonn, who many 
years ago paid great attention to the study of agates, always 
maintained that the liquid from which the silica was deposited 
gained access to the cavities through special openings, or inlets, 
of infiltration.! In some specimens the canal actually remains 
open, but usually it has become choked by continued deposition 
of silica. An agate may be so cut by accident that the section 
passes through this infiltration-channel, as in fig. 1, PI. II., 
where the original inlet is seen at a . In some specimens several 
openings of this kind may be detected. Assuming, however, 
that the solution of silica was thus introduced, it is difficult to 
see how the deposit could have been regularly thrown down in 
concentric layers all round the walls of the hollow ; no thicker* 
be it remarked, on the floor of the cavity, than on its roof. 
It is true we find in certain agates horizontal layers, as though 
the mineral matter had settled on the floor in obedience to 
gravity ; but then we are perplexed at finding that these flat 
bands often alternate in the same specimen with regularly con- di- 
centric deposits, which run with uniform thickness all round the v' : 
of Silica geologically considered.” By Professor T. Rupert Jones, F.R.S., 
&c. “ Proc. Geol. Assoc.” vol. iv., p. 443. 
* 11 Philosophical Magazine,” (3), vol. xxii. p. 213. 
t Ueber die Achat-Mandeln in den Melaphyren. Haidinger’s “Natur- 
wiss. Abhandlungen,” vol. iii. part i. pp. 93, 147. 
