514 



where the lapidifying process must have commenced at a num- 

 ber of separate points, so as to produce spherical or fusiform pe- 

 trifactions, independent of each other, in which the woody struc- 

 ture is apparent, while in the intervening spaces the wood has de- 

 cayed, having after removal been replaced by mineral matter. In 

 some petrifactions the most perishable, in others the most durable 

 portions of plants are preserved, variations which doubtless depend 

 on the time when the mineral matter was supplied. If introduced 

 immediately on the first comme cement of decomposition, then the 

 most destructible parts are lapidified, while the more durable do 

 not waste away till afterwards, when the supply has failed, and so 

 never become petrified. The converse of these circumstances gives 

 rise to exactly opposite results. As to the manner in which the 

 minutest pores and fibres discoverable by the microscope, even the 

 spiral vessels themselves can be turned into stone, or have their 

 forms faithfully represented by inorganic matter, no satisfactory 

 explanation has ever yet been offered. In considering, however, this 

 question, you will do well to consult the important suggestion which 

 a celebrated chemist, our late lamented Secretary, Dr. Turner, has 

 thrown out on the application of chemistry to geology. He reminds 

 us that whenever the decomposition of an organic body has begun, 

 the elements into which it is resolved are set free in a state peculiarly 

 adapting them to enter into new chemical combinations. They are 

 in what is technically termed a nascent state, the constituent mole- 

 cules being probably of extreme smallness and in a fluid or gaseous 

 form, ready to obey the slightest impulse of chemical affinity, so 

 that if the water percolating a stratum be charged with mineral in- 

 gredients, and come in contact with elements thus newly set free, a 

 mutual action takes place, and new combinations result, in the course 

 of which solid particles are precipitated so as to occupy the place 

 left vacant by the decomposed organic matter. In a word, all the 

 phsenomena attendant on slow putrefaction must be studied when- 

 ever we attempt to reason on the conversion of fossil bodies into 

 stone; and in regard to silicification, Dr. Turner has shown how 

 great a quantity of silex is set free as often as felspar decomposes, 

 and how abundantly siliceous matter may be imparted from this 

 isource alone to running water throughout the globe. 



As I have mentioned the name of Dr. Turner, I cannot pass on 



