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. Many of these nodules cannot be removed from the sand in which they are imbedded, being reduced to a soft smooth pulp, and some even of those specimens, which are now before the Society, might in their moist state be crushed between the fingers with the greatest ease.
Of hornstone I met with several varieties. One exhibits alternate bands of a smoky brown colour and white, with a glimmering lustre, shewing it to approximate to the nature of quartz; yet even in this the white bands are reduced more or less to an earthy consistence.
Another variety is the entrochital hornstone, and this appears to have suffered little if any change.
A third variety is the compact nodular hornstone, of a dull greyish white colour, and often intermixed with chalcedony. The external part of this, to a considerable depth, is reduced to a white earth; the interior is more solid, but even where the chert is but little changed the chalcedony in contact with it is totally disintegrated.
The fourth variety, and the most remarkable of any, appears to have been a madrepore agate, in which the organic part was converted into quartz, while the matter which connected the tubes was chalcedony or hornstone. In this state being subjected to the same violent friction as the other materials of the gravel, it assumed the common figure of a rolled pebble. It has however since that period
