1904-1905.] 329 



agates, or Scotch pebbles, were found all over the world on 

 sea-beaches and in river-beds, as water-worn pebbles. In 

 Ireland they were found on the South Coast, in County Wick- 

 low, and more rarely near the Giant's Causeway. In Scot- 

 land and in Germany they occurred also in their mother-rock, 

 which was generally an andesitic lava. Here the agates 

 occurred in steam cavities, once empty, but now filled with 

 agate, which had been deposited therein from an aqueous 

 solution of silica percolating the lava. Hence the embedded 

 agates and their banded structures preserved the contour of 

 the steam-bubbles, imprisoned at one time in their upward 

 ascent through the molten and boiling rock, as the latter con- 

 gealed into a solid. Agate, according to the mineralogist, was 

 a variegated chalcedony presenting many variations in colour 

 and structure. Each distinct variation gave rise to a class- 

 name such as " moss-agate," " eyed-agate," " onyx," '' sard," 

 " carnelian," &c. The variety in colour was a point easily 

 settled by chemical analysis, and it has been found that iron, 

 manganese, and titanium oxides were the chief colouring 

 agents in agate. The variety of structure displayed by the 

 concentric bands of an agate and its abnormal growths pre- 

 sented difficulties, some of which were yet unsettled. Many 

 years ago the German geologists discussed the question of 

 agate-genesis very fully, and many of them came to the con- 

 clusion that agates had been deposited from a hot solution of 

 silica traversing the mother rock shortly after its consolida- 

 tion from a liquid state. Noggerath, of Bonn, stated that 

 the siliceous solutions entered at isolated points in the sides of 

 the cavities; these he called "agate-tubes." Haidinger, on 

 the other hand, was of the opinion that the solutions were 

 secreted from all over the cavity-walls, and that the tubes 

 were really " tubes of escape." Ruskin was the first geologist 

 who studied the question of agate-growth very deeply. He 

 pointed out that agate was really a crystalline growth de- 

 posited from a hot solution of silica. He showed also that 

 two opposing forces of crystallisation were at work, one tend- 

 ing to build up hexagonal crystals and the other spherical 



