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POPULAR SCIENCE REVIEW. 
walls, as shown in fig. 2, PI. II. Bischof suggested that the 
horizontal bands were formed when the fluid, having been intro- 
duced rapidly, was then allowed to rest in the bottom of the 
cavity ; whilst the concentric zones were precipitated from a 
solution which filtered in slowly, and merely spread over the 
walls without accumulating on the floor.* To avoid the diffi- 
culty of explaining the formation of banded agates by admission 
of the liquid through special inlets, another hypothesis was 
advanced by the late Professor Haidinger. According to him 
the genesis of an agate could be best understood by assuming 
that, instead of a local infiltration, there had been a general 
exsudation through the walls of the cavity, so that all parts — 
the roof not less than the floor — would thus become uniformly 
coated with silica. The great objection to this explanation 
lies in the difficulty of understanding how the solution could 
continue to gain access to the cavity after the first imperme- 
able layer had been deposited. Most mineral substances are 
porous, and Bischof has cited the case of a compact basalt 
which when freshly broken was found to contain drops of water 
in the very heart of the rock. Some of the layers of agate are 
permeable with great ease, either through distinct pores or 
between the fibres of which chalcedony commonly consists, as 
shown under the miscroscope. But it will be presently seen, 
when referring to the method of staining agates, that whilst 
some layers are thus freely permeable others appear to be ab- 
solutely impervious ; and it is difficult to conceive how the agate- 
forming process could be continued after an impervious lining 
had once been thrown down upon the walls. To meet this 
objection, however, it is argued that every agate is sufficiently 
penetrated by direct fissures to offer means of ingress to the sili- 
ceous solution. Whatever views may he held as to the formation 
of the main mass of the agate, it is generally believed that the 
first lining of the cavity, in the form of a thin layer of delessite, 
or ferruginous chlorite, which constitutes the green rind of 
most agates, is the result of a general percolation and not of 
local deposition. 
As both theories obviously present difficulties, a third mode 
of origin has been suggested by Dr. Beusch.f If a thin cream 
of plaster of Paris be introduced into an irregularly-shaped 
cavity, shaken round, and then poured out, a layer will be left 
lining the walls of the hollow ; by introducing in this way 
plaster of various colours, successive layers are formed ; and, on 
cutting open the nodule, the appearance presented is strikingly 
similar to that of a section of a banded agate. Reusch supposes 
* “ Lebrbuch d. Chem. u. Pbys. Geologie,” 1866, vol. iii. p. 630. 
f Ueber den Agat. Poggendorffs u Annalen,” vol. cxxiii. p. 94. 
