383 Proceedings of Philosophical Societies. [Mat, 



Arborizations in chalcedony are of by no means unfrequent 

 occurrence. Sometimes they are so distinct as at once to command 

 attention ; but often, from the minuteness of their ramifications, or 

 from their being excessively crowded together, they are considered 

 as mere stains, and thus elude superficial observation. 



The whole may be divided into three classes. The first will 

 comprehend those which from their external form, and the perfec- 

 tion of their internal organization, will almost universally be ac- 

 knowledged by competent judges to te undoubted vegetables. Their 

 colour is generally green or blackish-brown, and they belong to the 

 families of conferva, lichen, and the musci. The second class 

 includes those which are invested with a crust of carbonate or oxide 

 of iron, and in which the structure can only be observed in the 

 transverse sections, and of which therefore the real origin is not so 

 certain as in those that belong to the first class. Their colours are 

 various ; red, dingy purple, a;nd ochre yellow, are the principal. 



The third class includes those in which the vegetable form and 

 structure are more or less perfectly imitated by grains and crystals 

 of chlorite, and by metallic oxides, but which, however accurate 

 the resemblance, are truly pseudomorphous. Their colours verj' 

 generally agree with those of the first class. 



The assistance of chemical agency was also had recourse to, for 

 the purpose of still farther confirming the reality of the distinctions 

 aboveraentioned. It was found that agatized wood was blackened 

 by immersion in sulphuric acid, but that no such effect was pro- 

 duced on chalcedony or on chlorite ; a certain number of speci- 

 mens of the first and third classes were then immei-sed in boiling 

 sulphuric acid, and the result was, that the fibres of real vegetable 

 origin became black, while those which consisted of chlorite did 

 not become black, but effervesced very sensibly. 



How did these vegetables become enveloped in a mass of chalce- 

 dony? It is well known that the confervae retain their green colour 

 only while they are alive ; and as in many of these specimens not 

 only the colour but the free unconstrained attitude of the plant is 

 perfectly preserved, the theory most consonant with actual appear- 

 ances seems to be, that the plants were involved in an aqueous solu- 

 tion of silex so dense as to be capable of speedily gelatinizing, and 

 in which they were preserved from further change wliile the con- 

 version of this jelly into hard stone was taking place. 



CORNWALL GEOLOGICAL SOCIETV. 



At a meeting of this Society, on the 9th of April, a p:iper was- 

 • communicated by Ashurst Magendie, Esq. upon tbe occurrence of 

 granitic veins in argillaceous schisius at Thouschole. At this point 

 of the coast he 0l)served the schistus to terminate, and the granite 

 to commence. The schistus is of a greyish colour, rather hard, 

 "but breaks in largo fragments in the direction of the strata : the 

 granite is of a fine grahi, and the felspar is of a light flesh colour, 

 «nd contains i)ut a small proportion of mica. At the junction uu- 



