AGATES, CARNELIANS, AND JASPERS. 159 
inner surface of which it begins to be deposited, coat upon 
coat uniformly following all the ins and outs of the sur- 
face, just in the same manner as the soot and dirt carried 
in through the leak in the roof uniformly stains or coats 
all the mouldings and irregularities of the plaster, in the 
case above referred to. In course of time a deposit of 
Chalcedony, also of uniform thickness, coats the inner face 
of the Zeolite Layer. As in both of the former cases, the 
deposition of the Chalcedony may go on without any im- 
portant interruption until coat after coat is laid on, and the 
whole cavity becomes completely filled. More often, in fact 
usually, a change arises at an early stage in this part of the 
process, and some new features of great interest arise. 
These may now be noted in some detail. 
One of these features is connected with the fact that 
the Green Earth does not adhere very firmly to the walls of 
the cavity. As a matter of fact, it is an easy matter 
to scrape the whole of it from the surface upon which it 
was deposited. This feeble adhesion sometimes leads to 
the Green Earth being detached during the ingress of the 
first Chalcedony solution. If we carry our illustration of 
the effects of a leaky roof a step further, and imagine that the 
plaster ceiling has been papered before the leak began, it will 
serve very well to illustrate the point in question. The rain- 
water soaking in gradually softens the paper of both the 
ceiling and the walls, so that it becomes detached in places, 
and hangs downward in tattered shreds and rags. Upon 
these ragged and torn shreds of paper the dirty water 
gradually makes its way down, eventually staining the paper 
as the water dries up. Imagine that process repeated many 
times, and it will be easy to understand how the rags of paper 
would become coated on all sides with quite a measurable 
thickness of the staining material. Indeed, if one could 
conceive of this lately-added coat being strong enough to 
bear its own weight, it would not be difficult to understand 
how it might come, in the course of long time, to be entirely 
coated with dirt. Instead of ragged strips of paper, spiders’ 
webs, hanging from the ceiling, would act just as well as 
surfaces of attachment for the dirt brought in by the 
leak. 
