198 Proceedings of the Royal Physical Society. 



under the present writer's charge in the Edinburgh Museum, 

 that the nature of the solutions carried in by osmosis varied 

 within wide limits, and that, too, often at close intervals. 

 When solutions of opal-silica were carried in, the particular 

 layer thus formed arranged itself as horizontally as the fluid 

 in a spirit-level. When solutions tending to become chalce- 

 donic silica followed, these deposited layers which conformed 

 at the base to the opaline layer, but, at the sides, climbed 

 the dome above, and coated that part as much as the parts 

 below or even more so. Solutions of intermediate composi- 

 tion comported themselves according as the one constituent 

 or the other predominated. It may be remarked here that 

 layers of horizontally-deposited silica are usually termed 

 Onyx — a name, it must be remembered, which may include 

 Opal, Chalcedony, and their modifications, and has reference 

 only to the parallelism of the layers, and not to their 

 composition. 



Many agates containing Opal bands have previously 

 developed " eyes." Where these are covered by Opal, sections 

 show that the eyes were simply submerged by the rising 

 deposit, which abuts against the eyes abruptly. But where 

 Chalcedony has encountered these prominences, it has 

 obeyed the usual law, and has enwrapped the eyes con- 

 formably to their shape, as already noticed. The difference 

 in comportment is very striking, and cannot fail to attract 

 attention in the case of such a collection as that above 

 referred to. 



Like the chalcedonic silica, the opaline form often includes 

 an admixture with zeolitic matter. This forms the true 

 Cachalong. 



Not uncommonly the central portion of an agate may 

 remain unfilled. When this is the case, the last- deposited 

 layers usually crystallise out as either Quartz or Amethyst 

 — probably because no opaline matter was present in the 

 last stage of growth. 



There is reason for believing that the consolidation of the 

 siliceous contents of an agate occupied a long time, and that 

 both the lower opaline layers and the Clear Chalcedony con- 

 solidated at an earlier stage than the agate layers, properly 



