preserved in Chalcedony, 515 



Impossibility of transmitting a ray of light through these specimens, 

 our sight of them is limited to that portion which happens to run 

 along the fractured or polished surface. Numerous red, white and 

 yellow agates are every where to be found among collectors and 

 dealers, in which the appearance of fibres is such as to render it 

 very probable that they are portions of vegetables incrusted by 

 oxides of iron, altbough the obscurity of the specimens must neces- 

 sarily render this uncertain until they are submitted to chemical 

 trials. If these are vegetable fibres there is no difficulty in account- 

 ing for this concealment of the vegetable by the metallic deposit, 

 since it is easy to understand how from a compound solution of 

 flint and iron in water, the iron might first be precipitated on the 

 vegetable, in the same way as we daily see it deposited in chalybeate 

 springs, and how the subsequent deposition of flinty matter might 

 involve and penetrate the whole. It is besides known to botanists, 

 that many plants possess the property of attracting from their state 

 of combination the earthy base of some of the salts which are dis- 

 solved in natural waters, a property which may be subservient to 

 some purpose in the economy of the plant, unknown to us. Such 

 is the case with Chara vulgaris, which is always found incrusted 

 with a coating of chalk or calcareous subcarbonate, and such appears 

 also to be the case with Byssus nivea. 



It is worthy of remark that in almost all the specimens of chal- 

 cedony which appear to contain aquatic confervsc, not only the 

 vegetable structure is perfectly preserved, but the plant, however 

 light and yielding its texture, is disposed in as free a manner as if 

 still living and floating in the water which was its native element. 

 Together wivh these circumstances, the natural colour is often 

 equally well preserved, and the various specimens of the confervas 

 in particular, which are the plants of most common occurrence, 



3 T 2 



