428 Mr. Aikin on the Gravel at Litchfield, 



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 gene- 

 rally 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 remark i le of any, appears to 

 have been a madrepore agate, in which the organic part was con- 

 verted 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 



