AGATES, CARNELIANS, AND JASPERS. 153 
in a lava; and the transporting agent has been, in all cases, 
water, percolating through the rock from above downwards. 
The aqueous solutions in all true agates have made their 
way into the cavity through its stony walls, and not through 
any chink or hole in the side. Lastly, the outer layers 
of the agate are usually the older, and those nearest the 
centre are those formed last. 
Agates are generally confined to rocks of one or two 
definite kinds. Asa rule, they occur in andesites, which 
are lavas containing but little potash, usually almost none; 
which, further, contain a fairly large percentage of felspar, 
usually the lime-soda felspar known as Labradorite, in 
association with which occurs a relatively small percentage of 
the ferro-magnesian silicate Augite. I know at present of no 
agate-bearing rock in which the mineral Olivine occurs, nor 
of one in which Quartz is an original constituent. Referring 
this to chemical composition, one may say that agates are 
rare (or perhaps never occur) in rocks in which the percen- 
tage of silica present in the unaltered rock rises about 50 
per cent., or falls below 45. Furthermore, it is only under 
special conditions, regarding the exact nature of which we 
probably have yet much to learn, that agates are developed 
even in the rocks of which the composition is suitable. As 
illustrative of this may be mentioned the fact that agates 
are of very rare occurrence, and, indeed, in their typical 
form, are unknown, in the Ordovician andesites of the 
English Lake District, Wales, and Scotland; nor do they 
occur in that form in the numerous andesite lavas of 
Carboniferous age in Scotland. Nor again, have they been 
recorded from the many andesite lavas which form a large 
number of the voleanic rocks now in process of formation 
in many parts of the world. These facts appear to suggest 
that there may be some factor concerned in the formation 
of agates whose precise nature has not hitherto been sus- 
pected, even by Dr Heddle, who did so much to advance our 
knowledge upon this particular subject. As this address 
proceeds, I hope to be able to indicate what the precise 
nature of that particular factor may be. But for the present 
there are several points which need a fuller explanation 
before we can deal with theoretical considerations. 
