1848.] HAMILTON ON THE AGATE QUARRIES OF OBERSTEIN. 215 
_ This account nearly corresponds with that which I had already 
received from Signor Pistrucci, the engraver to the Mint, and one of 
the most celebrated engravers of cameos and precious stones of the 
present day ; the only difference in the process consisting in the sub- 
stitution, according to Signor Pistrucci, of olive oil for honey, and 
that the immersion did not last so long. ‘This remarkable property 
of the agate appears to be owing to the different degrees of porosity 
of the different layers of the stone, whereby they are liable to be 
penetrated by colouring fluids in different degrees, the sulphuric acid 
carbonizing the vegetable matter already absorbed by the stone. 
There is only one further pomt respecting these Oberstein agates 
to which I wish to call the attention of the Society. The change of 
colour being owing to the porosity of the different layers, it is stated 
by Prof. Noeggerath that some species of chalcedony are found to be 
so porous that the minute hollows by which the stone is penetrated 
can be seen by means of a magnifying-glass ; they appear like bubbles 
either round or long, occasionally indeed being drawn out to a con- 
siderable length as compared with their breadth, and sometimes run- 
ning into one another, or as it were anastomosing. In general, how- 
_ ever, these hollows are so extremely minute that they are only visible 
with microscopes of very high power. The real fact is, that under a 
very powerful microscope, those agates which are generally supposed 
to be of an amorphous uncrystalline structure, do exhibit a series of 
concentric rings parallel with the outer circumference, each of which 
is composed of a congeries of minute radiating fibres at right angles 
to the rings or bands of colour, the incipient germs of crystallization 
invisible to the naked eye, and resembling what the Germans appro- 
priately call faserig, such as we see in fibrous gypsum and in stalac- 
tites. It is no doubt through this fibrous structure that the fluids 
penetrate by which the colour of these stones is artificially altered. 
The running together of the hollow cavities, described by Prof. 
Neeggerath, is evidently another phenomenon, and closely resembles 
the anastomosing process of the vascular structure, as described by 
Mr. Bowerbank in the paper above quoted, where he also describes 
other specimens from Oberstein without any anastomosing appear- 
ances, but exhibiting numerous long and thread-like fibres representing 
the radiating appearance to which I have just alluded. Many too of 
the agates so examined under the microscope had no doubt been artifi- 
cially coloured, and thence may possibly have been derived the colour- 
ing matter described by Mr. Bowerbank, if indeed any colour can be 
fairly distinguished in an object examined with a power of 800 linear. 
In concluding these remarks, I do not wish to be understood as 
denying the possibility of flint stones and chert, or even the red cor- 
nelian veins from the conglomerate beds of Oberstein, containing re- 
mains of organic structure; but as I cannot agree with Mr. Bower- 
bank’s theory of attributing all flint stones and chert to a spongeous 
origin, | am bound to protest against its applicability im any degree 
to the agates of Oberstein. 
January 16, 1848. 
Since reading the above paper I have had an opportunity of ex- 
