od 
AGATES, CARNELIANS, AND JASPERS. 173 
to reduce the whole to a dull, chalky mass, almost like the 
white outer rind of a chalk flint. 
Exposure to extremes of temperature brings about a 
separation of the various layers of an agate. At the same 
time the same agent, probably co-operating with the effects 
of surface-water, tends to devitrify the colloidal part. Both 
of these causes help to bring about the disintegration of the 
agate, and to aid in its final return to the state of solution. 
True Agates, being formed in closed cavities, consist only of 
such materials as can make their way in solution through the 
inter-molecular spaces of the rock. No mechanically-sus- 
pended materials can possibly find an entrance under such con- 
ditions as these. Hence Jasper, which is Chalcedony rendered 
opaque by ferruginous impurities, and which occurs in some 
agate-like structures, is perhaps never found in a true agate. 
Where fissures exist in the rock in which agates are in 
process of formation, various substances may be carried in 
suspension into the chink, and be there deposited. Thus 
fine clay, often charged, too, with iron, may mingle in various 
proportions with solutions which contain the materials of a 
pure agate. Such mixed materials are, of course, deposited 
in bands whose surfaces are parallel to the walls of the fissure. 
Amongst such compounds are sure to be found every gradation 
from pure Chalcedony, through Carnelian in one direction, to 
Jasper ; and, in another direction, through impure Carnelian 
into Chert. In a third direction the same materials may 
incorporate with one or other of the Green Earths. No 
better example of these Vein Agates can be found than the 
beautiful specimens from Burn Anne near Galston, or from the 
Campsie Fells, and also from the Garleton Hills, of which fine 
specimens are exhibited in the Scottish Mineral Collection. 
A word may be added regarding Onyx :—Agates are perme- 
able in different degrees by various solutions which make their 
way readily along certain bands, but cannot find an entrance 
into others. Oil, solutions of sugar, honey, and other carbon 
compounds, will soak into certain bands in agates, and if, after 
being thus permeated, the agate is steeped in sulphuric acid, 
charring will occur, and the onyx of jewellers is the result. 
In conclusion, I may advise those readers who are desirous 
VOL. I. 14 
