12 MR GOODCHILD. 
AGATES, CARNELIANS, AND JASPERS. 
By Mr GoopcuiLp, H.M. Geological Survey, F.G.S., F.ZS., 
Curator of the Collections of Scottish Geology and 
Mineralogy in the Edinburgh Museum of Science and 
Art, Past President of the Society. 
(Read 4th May 1899.) 
THERE are various prevalent misconceptions regarding the 
nature and mode of origin of Agates. Foremost amongst 
these is the common idea that they are of the same nature 
as the stones in a conglomerate, an error which the title 
“ Pebble,’ so commonly applied to them, has done much to 
perpetuate. Next to that, perhaps the commonest miscon- 
ception amongst those who realise something of the true 
nature of an agate, is that they represent materials carried 
into cavities through one or more small holes in their side. 
A third idea of an erroneous nature in regard to agates is, 
that they have been formed in somewhat the same way as 
amber, except that they inclose fragments of moss or of 
other plants, instead of flies. 
Instead of making any further reference to these popular 
fallacies, it may be as well to state here at once, that agates 
are purely inorganic in origin; that they are not in any way 
due to causes of a mechanical nature as true pebbles are; 
that they are of later date than the rock in which they 
occur; that they have been filled by materials in solution, 
which have made their way into perfectly closed cavities 
through the solid rock by which those cavities are 
surrounded ; and lastly, that they never enclose organic 
bodies. 
Putting these statements in yet another form, it may be 
said that agates consist of chalcedonic matter, derived from 
the silica of rocks which formerly occurred above the point 
where the agates are now. . The materials have been carried 
into pre-existent cavities, which were formed by the libera- 
tion of gases or vapours in a cooling eruptive rock, usually 
a ae 
