1848.] HAMILTON ON THE AGATE QUARRIES OF OBERSTEIN. 213 
electrical causes, by which the siliceous particles have in the softer 
beds been segregated from the mass and collected together in the 
vesicular hollows, thus forming the agate nodules, while in the other 
and harder beds these siliceous particles remained disseminated 
throughout the whole mass, thus giving them a greater degree of 
hardness and uniformity. 
The agate nodules themselves vary much in character, colour 
and substance; the smaller ones are generally completely solid, the 
whole cavity being filled up with the compact chalcedonic mass, ge- 
nerally of an uniform pale ash-grey colour ; those of a larger size are 
more frequently veined with layers or bands of different colours, and 
are invariably hollow, the outer circumference consisting generally of 
the same pale grey chalcedony as in the smaller nodules, varying 
more or less in the coloured bands, which however are not always at 
once perceptible ; this outer portion varies in thickness from a 
quarter of an inch to about an inch and a haif; the interior is gene- 
rally lined either with botryoidal mammillations, or with imperfect 
quartz crystals, which sometimes assume a bright amethystine colour. 
It is only the outer or compact portion of the nodule which forms 
the real agate, the rest being of too brittle a nature to bear polishing, 
unless when occasionally perfect crystals are found. Some portions 
of these chalcedonic agates, which have undergone a slight degree 
of decomposition from the effects of exposure to the weather, show 
the original mammillated structure of the successively-formed. bands 
or layers, superimposed on one another in an inverted or contrary 
order to that in which they were formed. The great proportion of 
these agate nodules are however unfit for any purposes of trade, in 
consequence of the thinness of the outer portion. 
One remarkable feature of these agate nodules, and which marks 
an important difference between this rock and the usual class of amyg- 
daloidal traps, including even the neighbouring rocks of Oberstein, is, 
that in most cases, and particularly when the nodules are of any con- 
siderable size, they are found to be compressed, flattened out, and 
as it were elongated. This peculiarity is important when we come 
to consider the origin and formation of the agates, inasmuch as, in 
connexion with the large size which some of. them attam, it would 
seem to preclude the idea of the siliceous particles having been de- 
posited in pre-existing vesicular cavities caused by the expansive 
power of gases during the consolidation of the igneous rock, in the 
same manner as the cavities found in some kinds of vesicular basalts 
and trap rocks are filled up: are we not rather led to the conclusion, 
that the hollows in which the agates are now found were caused by 
the molecular aggregation of the siliceous particles compelling the 
surrounding matter to yield in proportion to the mtensity of the 
attraction of these homogeneous particles ? 
On the other hand, the still existing hollows in some nodules, and 
the concentric nature of the bands of siliceous matter which lines the 
surface of these cavities, bearing such evidence of the deposition of the 
outer prior to that of the inner layers, prove that at the very period 
when the deposits were first commenced, the cavities had generally 
assumed the form and shape which they now retain. Perhaps how- 
