Arthur Aikin

“ Of the former variety the most abundant is the common nodular agate, or Scotch pebble. In these the central nucleus and other parts composed of pure quartz are unaltered, the flesh-coloured zones, consisting of silex and red oxid of iron, have become more or less adherent to the tongue, have nearly lost their lustre, and have had their hardness much impaired; while the milk-white zones which in the perfect state of the substance were of pure chalcedony, are in every instance reduced to an opake white earth, yielding generally to the nail, and strongly adherent to the tongue. ”

“ The pebbles of quartz, which are numerous, appear to have undergone no change whatever.
The pebbles of limestone are the most abundant of all, and have undergone considerable change. The madrepores and other coralloidal bodies of which the limestone is principally composed, resist the solvent action of the water which percolates through the sand much better than the compact calcareous matter that is interposed between them does; the same is the case with the slender veins of calcareous spar by which the limestone is traversed ”

“ The moisture contained in the bed is primarily rain water, and it is not easy to see what active agent it can become charged with in draining into the interior of the mass, except carbonic acid or carbonate of lime: any of the stronger acids, such as the sulphuric, resulting from the decomposition of pyrites, would immediately be neutralized by the calcareous matter in which the whole bed abounds. Alternations of moisture and dryness, of heat and cold, to which the decomposition of the exposed surfaces of rocks is so generally attributed, could have little or no influence in this case ”