214 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 19, 
ever the wavy and broken lies which these concentric bands often 
display, assuming almost the contortions seen in the old altered rocks, 
may be the evidence that these vesicular hollows did undergo very 
considerable alterations of form, bemg sometimes contracted in one 
direction, or expanded in another, after the deposition of the first 
laminze, and during the gradual formation of the nodule itself and 
the deposition of the inner layers. | 
The alternation to which I have already alluded, of the soft beds 
containing the agates with the harder compact beds in which no 
such cellular cavities exist, would seem to point to some important 
change having taken place subsequently to the consolidation of the 
beds, and may perhaps tend to the confirmation of the idea that these 
cavities, if formed during the cooling of the beds, must have been 
altered in their shape by pressure either previously to the deposit of 
the chalcedonic matter, or during its gradual formation. 
The solution of this difficulty, and of the apparent contradiction 
which it involves, is well-deserving the attention of geologists, and I 
am happy in being able to state that Professor Neeggerath of Bonn 
is now particularly directing his attention to the full examination of 
all the pheenomena connected with the agates of Oberstein. 
I have already alluded to the fact of the agate-workers of Ober- 
stem having learnt from Italian artists the means of artificially 
changing the colour of the agates. In some cases the ash-grey layers 
of the agates of Idal become, under this treatment, of a dark brown or 
chocolate colour, and in a few fortunate imstances where the arrange- 
ment of the strata is appropriate, and the different bands are suffi- 
ciently strongly contrasted, the effect of the treatment is to produce 
alternate black and white, or brown and white layers, resembling the 
onyx, and even sardonyx. In fact, it appears probable that all the 
onyxes of the present day are the result of this treatment, and there 
is good reason to believe that not a few of those which have come 
down to us from ancient times were produced in the same way. 
In confirmation of this view I must refer to an interesting paper by 
Professor Neeggerath, published in the ‘ Neues Jahrbuch,’ No. 4, for 
1847, entitled, “The art of colourmg Onyxes, Cornelians, Chalcedonies 
and other similar stones, in explanation of a passage in Pliny.” It 
is here stated that the present mode of colourmg agates, and thereby 
producing very beautiful onyxes, is an art which has been long known 
to the lapidaries of Italy, but the knowledge of which has only been 
introduced into Oberstei within a few years. This agrees with the 
information which I myself received at Idal. The process is de- 
scribed as follows :—The stones being first well-washed, are placed in 
honey and water in a clean earthen vessel ; this is set in the ashes, or 
on a warm stone, but must not be allowed to boil, and the agates must 
be kept always covered by the fluid for a fortnight or three weeks. 
When taken out they are cleaned and placed in another vessel in sul- 
phuric acid, by which they must also be completely covered. This 
vessel should be covered over with a slate and also placed amongst 
hot ashes. Some of the softer and more porous stones or layers are 
coloured in a few hours, others require several days, while some are 
not at all affected by the sulphuric acid. 
