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ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixxi 
infiltrations ceasing, carbonate of lime, iron in some form, or other 
minerals, the results of the percolation of different solutions into the 
cavities, became crystallized in the hollows still left. 
While some agates show concentric bands of chalcedonic matter, 
entirely filling the cavity, the original sides of the vesicle or hollow 
evidently giving the form, however modified in parts, to the general 
arrangement of the layers, others, commonly termed onyxes, have a 
system of flat layers, parallel to each other, and considered to be 
also found parallel to the main bedding of the rocks in which they 
are discovered. In the one case the inside of the cells or cavities has 
been coated without much regard to gravity, while in the other the 
deposit more accords with a deposit having reference to it. In some 
agates we find that the cavity must have been filled m one way at 
one time and in another manner afterwards ; sometimes the one kind, 
sometimes the other, having been the first formed. Agaim, some 
cavities im igneous rocks have been found partially filled with parallel 
layers of chalcedonic matter, with small pendent stalactites of the like 
substance depending from the upper part of a hollow still left. 
The change of colour produced artificially in the agates by the 
workers in them at Oberstein, an art learnt from the Italians, and to 
which Mr. Hamilton calls attention in his communication, stating his 
belief (referring also to the labours of M. Noeggerath) that not a few 
of the onyxes which have come down from ancient times were thus 
treated, is of much interest mineralogically, since it shows the very 
different porosity of different layers in the agates, the least porous 
bands not being necessarily the nearest to the centre, but dispersed 
irregularly through the mass. To this porosity Mr. Hamilton calls 
attention, citing the researches of M. Noeggerath, who states that in 
some layers the minute hollows can be seen by means of a magnitfy- 
ing-glass ; that while some are round others are long, and that they 
sometimes run into one another. These hollows Mr. Hamilton con- 
siders may form interstices between the radiating crystals. By im- 
mersion for some time in honey and water, or olive oil, so that the 
pores of the agate become more or less filled with a substance to be 
carbonized, a subsequent soaking of the stone in sulphuric acid pro- 
duces a difference in the tints of the agate according to the porosity 
of the layers the most porous becoming black, while the least porous 
remain white or uncoloured. By immersion in a solution of sulphate 
of iron and a subsequent heating of the agate, a carnelian-red is in 
like manner obtained for the most porous layers, the iron bemg con- 
verted into a peroxide, while the least porous layers continue un- 
changed in colour. It would be out of place further to dwell upon 
the infiltration of mineral matter in solution into the isolated cavities 
of rocks. ‘Fhe mode in which the various minerals occur is highly 
interesting, as also their connection with the matter filling veims and 
fissures in adjoining parts of the same or adjacent rocks, as for ex- 
ample the fillmg of the fissures in the red conglomerate by the same 
kind of siliceous matter which entered into the cavities of the igneous 
rocks of Idal, the layers having in both cases adjusted themselves to 
the surfaces on which they were accumulated. 
