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solutions. They were often concentric, generally curved, seldom in 

 straight lines ; sometimes in beautiful folds, at others angular like the 

 bastions of fortifications. Not less varied than their forms were their 

 colours. How were we to account for the successive differences in the 

 molecular constitution of the agatescent matter ? A portion of every 

 agate consisted of soluble or opal silica. Was it the varying strength 

 or heat of the solution out of which it was formed which caused these 

 differences of crystalline structure ? Was it the intermittence of sup- 

 ply of fluid injected or infiltrated into the cavities ? Dana laid great 

 stress on this as the cause of banded structure. Mr. Ruskin, in his 

 beautiful and exquisitely-illustrated memoir on Agates, in the fifth 

 volume of the Geological Magazine, attached little importance to it. 

 He saw rather in the agate a battlefield of crystalline energies ; these, 

 however, modified to some extent by conditions of deposition. And 

 truly, when we looked at these pisolitic, concentric, and hemispherical 

 depositions of chalcedony in the Beekite and marked the circular and 

 semi-circular lines of ths agate, and often, too, the tiny nodular 

 concretion in the centre of these, we could not but allow how much 

 crystalline energy was involved in the structure of a banded agate. 



On the other hand, there were two specimens before them in 

 which the original orifices of infiltration were still apparent, and in 

 which, too, it seemed that periods of intermittence in supply, strength, 

 chemical constitution, perhaps temperature, of infiltrated or injected 

 liquid, are strongly marked. In one of them, also, the action of gravity 

 in determining the direction of the parallel lines seemed undeniable. 

 The porosity of agates was also remarkable. Most that were met 

 with, 99 out of every loo, were in some manner coloured. Generally 

 the agate was boiled for some time in a solution of sugar. This being 

 absorbed by it, the stone was put into oil of vitriol— strong sulphuric 

 acid— which decomposed the sugar, and, by depositing the carbon in 

 the pores of the pebble, gave it the dark colour which was supposed to 

 be admired. By dyeing them blue, the generally honest Swiss was 

 enabled to palm off a piece of coloured agate on the unsuspecting 

 traveller as a genuine bit of lapis lazuli. 



Having spoken so much of opal-silica, he could not leave the sub- 

 ject without reference to it in its purest natural form. It might be defined 

 as amorphous translucent silica, with resinous fracture and essential 

 water. It was never pisolitic or reniform in structure. Distinguished 



