194 Proceedings of the Royal Physical Society. 



silica, theoretically considered as Quartzine, but whose 

 precise mineral nature does not yet appear to have been 

 satisfactorily determined. 



To sum up, so far : — Three layers may be deposited within 

 an agate, which follow each other in definite order — Cela- 

 donite, Zeolite, and Clear Chalcedony — and each of which 

 as a rule, uniformly coats the surface formed by its prede- 

 cessor. We shall do well to follow Dr Heddle in terming 

 these three layers collectively as the Skin of the Agate. 



Sometimes it happens that the Clear Chalcedony Layer 

 has continued to be deposited layer upon layer to a thickness 

 so great that Gravitation began to turn the balance against 

 the opposing force of Surface Tension. Then, very much the 

 same thing began to occur as would happen if several coats 

 of paint were to be laid one upon another upon a ceiling, 

 each coat before the former one had begun to dry. The 

 Celadonite began to sag a little here and there ; and once it 

 yielded at any point, any subsequent deposit, while yet in 

 the gelatinous condition, tended to elongate the pendant 

 portion under the influence of gravitation. In a few cases, 

 notably in Faroe, and at Norman's Law, in Fife, coat upon 

 coat of silica-jelly had been deposited in solution, and 

 stalactitic masses, pendant from the roof of the cavity, or of 

 many projecting points elsewhere, have been formed. It was 

 in this way that Stalactites in Agates have originated. Now, 

 Surface Tension is increased proportionately with increase 

 of surface. It therefore follows that, as the Stalactites 

 lengthened, they offered an increased surface as compared 

 with that of any similar basal area in other parts of the 

 cavity. More silica, therefore, was deposited near where 

 there were Stalactites than anywhere else within the cavity ; 

 and, for the same reason, more was deposited in the space 

 between two contiguous stalactites than was deposited else- 

 where. Hence, notwithstanding the advantage that Gravita- 

 tion had at first in its conflict with Surface Tension, the 

 latter force eventually gained the ascendancy, and accom- 

 plished most in the end. We shall presently see that a 

 consideration of the effects arising from this contest between 

 Surface Energy and Gravitation affords us the clue to under- 



