AGATES, CARNELIANS, AND JASPERS. 169 
the septum into the agate in the crystalloidal form may soon 
after have become a second time a colloid, and have comported 
itself within the agate cavity accordingly. 
The whole subject still awaits further inquiries, notwith- 
standing the numerous and prolonged investigations made by 
Dr Heddle and others. The broad facts are clear enough, 
explain them how one may :—A mixture of crystalloidal and 
colloidal silica has been carried in somehow under one set 
of conditions, and has formed the Chalcedony, while under 
another set of a different kind, silica, which has com- 
ported itself exactly as colloid silica would have done, has 
been carried in and has formed the Opal layers. Probably, 
as already stated, slight differences in the alkalinity of the 
solutions have been the determining factor in the process 
in the first instance ; while subsequent molecular changes may 
serve to account for some of the anomalies of later date. 
As a rule, the Opal layers in an agate are amongst those 
earliest formed after the Clear Chalcedony Layer and the 
Eyes, but there are, however, some few exceptions to this 
rule. But it is quite clear that, in many instances, there 
were repeated alternations of both forms of Silica; and, further, 
that nearly the whole process of agate-formation has been 
repeated within the same agate. 
We have now to consider a feature present in many agates 
whose origin and significance have given rise to considerable 
difference of opinion. This is the so-called “tube of entry ” 
of the earlier writers on agates, and the Tube of Escape of 
Dr Heddle. Without entering into any discussion of the 
respective merits of these various theories, I may state here 
the explanation that appears to me to best accord with the 
facts :—During the filling of an agate all the chalcedonic 
Silica jelly carried in, except that which formed the Clear 
Chalcedony Layer, remained in a coagulated, or at least, only 
partially-consolidated condition. The process of filling was 
continued in most cases until the entire chamber was quite full. 
At that stage the greater part of the nascent agate appears to 
have consisted of coagulated silica jelly—possibly with some 
of the Quartzine already crystallised—and surrounded by 
rock containing the same jelly in a highly diluted condition ; 
