AGATE PEBBLES FROM THE SOUDAN. 369 



alternately stewed and roasted in the hollows which served to collect 

 the scanty rainfall in the region in which they were obtained. 



It is interesting to observe the selective action of the solvent 

 agent in these specimens ; some layers and particular spots in the 

 pebbles have been scarcely touched, other portions have been deeply 

 eaten into. While to some extent this may be due to slight differences 

 in composition, it seems on the whole to be more likely to be in 

 consequence of differences in the consolidation of the originally 

 colloid silica — differences in the arrangement of the constituent 

 atoms, which have caused some parts to yield more readily than 

 others to the solvent action of the alkaline solution. 



I omit here all discussion of the possible agency of humic acids, 

 because the salt to which the solvent action is attributed is capable 

 of doing the work, and because it is probably abundant in the arid 

 region referred to, while prima facie we might expect the contrary 

 to be the case with regard to humic acids. 



DlSCTTSSION". 



The President remarked that the question whether solutions 

 of sodium carbonate are likely to be found in the Soudan required 

 further research. He corroborated the statement as to the quantity 

 of this salt among the basalts of India, and had seen similar eroded 

 agates in that country. It should be considered whether any other 

 agent could produce similar effects. 



Mr. Whitakee referred to the banded flints of south-east England 

 as presenting somewhat similar phenomena, although not acted upon 

 by sodium carbonate ; but the erosion was less marked than in 

 the Soudan pebbles. The differences in the bands of the flints are 

 molecular. 



Eev. A. Irving bore out Mr. "Whitaker's remarks on banded flints, 

 and referred to Eammelsberg's researches on the action of solvents 

 on different portions of the same mineral with variations of molecu- 

 lar structure. The humus-acids furnished by the desert-scrub he 

 regarded as the more likely solvent. 



Mr. De Eance had seen agates in this country eroded by 

 the action of blowing sand. He also alluded to the ridges produced 

 by wind-action in the laminated rocks of the Lake-district volcanic 

 beds, and alluded to the possibility of the Soudan pebbles being 

 acted upon in this manner. 



Sir Warin&ton Smyth instanced the existence of groovings in 

 the rocks of the parish of St. Just, and in St. Agnes, Scilly, which 

 had been formed by sand driven by wind, in the latter case to the 

 depth of 6 feet. He referred to the advantage taken by the Eomans 

 of their knowledge of the physical differences of banded pebbles to 

 heighten artificially the colour of onyxes. Whilst supposing that 

 some erosion might be produced by sand, he considered that in 

 jN'ubia, where the soil was impregnated with salts, these would 

 heighten the effect. Amygdaloidal rocks remained to be discovered 

 i)i situ in the Soudan. 



2c2 



