AGATES, CARNELIANS, AND JASPERS. 167 
striking indeed, and especially where alternate Opal and 
Chalcedony are deposited over an eye. At this distant 
interval of time since they were formed the difference 
between the two forms of silica cannot always be satis- 
factorily made out by either optical or chemical analysis, 
yet the well-known behaviour of solutions tending to deposit 
crystalline matter, as compared with others destined to become 
colloids, seems to confirm the view that the horizontal bands 
were once the colloid Opal, and those which coat the roof 
and sides equally with the floor were (as they still are) the 
hemi-crystalline Chalcedony. 
It occurs to me that there is just a bare possibility that 
there may be another explanation of this interesting pheno- 
menon. Most agates occur in andesites which were formed 
during periods of exceptional aridity. It may, therefore, 
be the case that the horizontal bands mark periods when 
the solutions of silica jelly were somewhat more diluted than 
usual, and the chalcedonie layers may have been deposited 
during the periods of greater drought. This would be very 
interesting, if it could be proved; but I must confess that 
I have many reasons for doubting whether it is really the 
case. So far as I can make out, the Scottish agates were 
formed during a temporary period of humidity, after the 
voleanoes had died out, and had already suffered much waste 
by surface agencies. They were certainly mostly formed well 
before the time when the Upper Old Red Sandstone was 
being laid down. I should be inclined to refer the date of 
most of them to that of the Orcadian Old Red, which is 
intermediate in age between the Caledonian Old Red (to the 
andesitic lavas of which agates are chiefly confined in Britain) 
and the Upper Old Red. In the Cheviots and elsewhere 
agates occur quite commonly in the conglomerate of the last- 
named rocks; which, of course, determines their age very nearly. 
In many agates there is a very remarkable alternation of 
the Opaline layers and the bands of Chalcedony. It is custo- 
mary to speak of all those which lie horizontally collectively 
as Onyx. This term, thus employed, may embrace both 
Chalcedony and Opal, but the former only when it conforms to 
a surface already horizontal, as those of Opal invariably are. 
These Onyx bands are of value to the geologist as indi- 
