134 THE ORIGIN AND HISTORY OF 



seen in loaves of bread that have been over-risen. These cavities 

 are subsequently filled with silica, sometimes in a crystallised, and 

 at other times a crystalline, an amorphous, or a colloid condition, 

 and resulting in the production of different mineral constituents. 

 That the cavities were thus formed is proved by the fact that they 

 are elongated generally in one direction — that is, in the direction 

 of the flow, instead of being globular or pear-shaped, with the 

 smaller end downwards, as indeed some are where the flow has 

 not been rapid. Some Continental writers who have been reluc- 

 tant to acknowledge the igneous origin of basalt and trap-rocks 

 have contended that the cavities have been pre-occupied by 

 crystals of calc-spar and other minerals, which on decomposition 

 have left vacuities which have afterwards been occupied by the 

 agates. 



2. — The Source of the Silica. — One theory is that after the 

 rocks were consolidated, siliceous waters, supplied from deep- 

 seated sources, as in the case of geysers and other hot springs, 

 permeated them and gradually deposited silica in the cavities. 



According to another theory, water containing carbonic acid 

 dissolved the felspar from the rocks and deposited the silica thus 

 set free in the cavities. And a third theory premises that also 

 from the rock itself silica was derived by a process of sublimation 

 produced by heat. Others have supposed that the mineral matter 

 has been conveyed by water from superincumbent rocks, as is the 

 case with the calc-spar in the Derbyshire Toadstone. 



That the silica is derived from the strata which contains the 

 agates is pretty certain, for the more decomposed the traps are, 

 the richer, better, and more consolidated are the Agates. 



3. — We must now consider the mode of entrance, deposition, 

 and arrangement of the various forms of silica in the cavity. 

 Some agates are cut so as to exhibit an apparent opening into the 

 cavity at one end, and sometimes in one or two other places. An 

 inspection of such a stone will show that the circumferential bands 

 do not meet at the smaller end, but run out on each side, forming 

 a botde-neck kind of channel. Through this channel or orifice it 

 has generally been supposed the silica- or agate-forming material 

 entered and spread itself by some unexplained method around the 

 sides of the cavities, in layers of different kinds and colours, as 



