514 Dr. MacCullochow Vegetable remains 



which occur in carnelian, and (though much more rarely) in the 

 agate nodules of trap. These are of a colour sometimes ochry yel- 

 low, sometimes white, and sometimes brown. Their forms, so 

 similar to the chlorite fibres, would induce us to class them together, 

 and it is probable that they actually consist of this substance, having 

 undergone some decomposition by which the green colour has be- 

 come brown, from affections of the metallic colouring matter. It is 

 at present impossible to account for this very singular disposition of 

 the chlorite. In some cases it evidently forms the centre of a fine 

 stalactite, of which the minute ramifications, afterwards preserved 

 in further additions of chalcedony, put on the appearance well 

 knov.n in the green stained chalcedonies of Faroe. But that it is 

 not necessarily stalactitical is certain, from its assuming the very 

 same disposition when existing in quartz crystals. It is probably 

 the result of a species of arborization, that obscure circumstance in 

 crystallization which appears to depend on a sort of polarity be- 

 tween distinct crystals, or throughout the interrupted parts of the 

 same one. Thus water on freezing, and various metallic substances 

 on crystallizing, ramify in certain directions. Thus it will often be 

 found that fibres of mesotype will hold their straight and radiating 

 course across stilbite or other associated minerals, the continuity of 

 direction in the portions of any crystal remaining unchanged, how- 

 ever the crystal itself may be interrupted by obstacles occurring in 

 its course. The same appearance may frequently be observed in 

 quartz and other crystallized substances, and it affords among the 

 various phenomena of the obscure process of crystallization, not 

 the least curious subject of inquiry. 



It is in the transparent chalcedonies that the fibrous structures 

 ;ire most visible, but they are also of common occurrence in the 

 opaque ones or agates, as they are usually called, although from the 



