158 MR GOODCHILD. 
of percolating water. We may speak of these collectively 
as the Green Earths, seeing that they are somewhat earthy 
in character, are not quite definite in their chemical com- 
position, and are usually green of some sort or other. A 
common Scottish variety of these Green Earths was identi- 
fied by Dr Heddle as Celadonite. In the case of most 
(perhaps of all) Scottish agates, these Green Earths are the 
first materials to be carried in. Usually they uniformly line 
the whole of the interior of the cavity, but the priming 
is of variable thickness in different cases, and it appears 
to be reduced almost to nothing in some localities, while 
elsewhere it may go on until the cavity is quite filled. 
It commonly happens that the conditions for the formation 
of the Green Earth Layer cease at that level within the 
rock at an early period, by which time another kind of 
solution, due probably to the decomposition of part of the 
felspars, has worked its way down to the level just before 
occupied by its predecessor. Then these solutions begin to 
percolate through the rock, through the walls of the cavity, 
and thence to the inner face of the Green Earth. Here a 
second deposit takes place. Like its predecessor it may be 
very thin or almost absent, or its deposition may be con- 
tinued for a long time, or may even go on until the whole 
cavity is filled. Usually this mineral differs from the Green 
Earth in that it passes, at an early stage, into the crystalline 
condition. Its nature varies within wide limits, but chemi- 
cally it is always a hydrous non-magnesian silicate, and, 
mineralogically it forms one of the family of Zeolites. 
After a time the solution from which the Zeolite Layer 
is deposited gradually follows its predecessor to a lower 
level, and the conditions which gave rise to that special 
kind of solution are hardly ever even temporarily repeated, 
and then only to give rise to a sporadic deposit. 
At this stage, the weak solutions of silica jelly which 
have been slowly making their way down through the solid 
rock in the rear of the former two begin to arrive at the 
level just before occupied by the solution from which the 
Zoolite layer was deposited. Then the jelly begins to trans- 
fuse through the walls of the cavity, through the priming 
of Green Earth, and through the Zeolite Layer, upon the 
