GEOLOGICAL SURVEY OF THE TERRITORIES. 371 



mode of decomposition of the woody tissue explains the various appear- 

 ances remarked in diiferent kinds of fossil wood after complete minerali- 

 zation ; for it is easy to understand that, after this softening process, 

 the wood is easily penetrated by different mineralizing elements, of 

 which the water or the imbedding substances are impregnated. Some 

 substances, especially lime and silica, do not destroy the woody tissue 

 in penetrating it. The internal structure of fossil trees of this kind can 

 therefore be studied in obtaining their lamellae, and observing them with 

 the microscope. This has furnished the means of exactly determining, , 

 if not the species, at l^ast the genera to which are referable fossil trees 

 of various geological epochs. 



Deposits of fossil wood of this kind are generally formed in connection 

 with our tertiary and cretaceous formations, and also, though more 

 rarely, in our carboniferous measures. There is a deposit of silicified 

 trunks in Southern Ohio, from which splendid specimens have been ob- 

 tained. Silicified trunks of the tertiary are now found strewn on the 

 sandy surface of the land in Mississippi, Arkansas, and farther west. 

 This mode of petrification has happened to trees in a standing position 

 as well as to prostrated trunks. In the first case the petrification of 

 the trees is generally homogeneous or unmixed. In prostrated trunks 

 the difference in the decomposition of the trees, at the time when they 

 were buried, has apparently varied the mode and the agents of minerali- 

 zation in such a way that, in the same specimens, part of a trunk is im- 

 pregnated with lime, while other parts are silicified, and still others 

 penetrated with oxide of iron or hardened into clay. Trees fossilized in 

 that way have been remarked at various places, especially in beds of 

 sandstone of the coal formations near Gallipolis, Ohio. At first it seems 

 difficult to explain how trees can have been preserved in a standing 

 position for a time long enough to produce mineralization. But in 

 examining the geological strata of our coal measures, for example, it 

 becomes evident that at repeated times, areas, sometimes of wide extent, 

 have been subjected to great depressions, and been covered more 

 or less rapidly with water and with the materials brought with it, espe- 

 cially sand. In that way entire forests have been buried, their trees 

 imbedded to a certain height and petrified in that position, sustained as 

 they were in their softened state by the imbedding materials. Such 

 fossil standing trees are of no rare occurrence in the sandstone of our 

 coal measures, and sometimes, as for example near Carbonclale, Penn- 

 sylvania, the miners have pierced their tunnels through such forests of 

 stone and brought to light an immense amount of their debris. 



It is questionable if, by the petrification of trees by silex, the silica 

 which has penetrated them was naturally in solution in the water cov- 

 ering the deposit, as Professor Gceppert will have it ; or if, according to 

 Professor Schimper's opinion, the silicification has happened only under 

 the influence of hot springs, impregnated with a large proportion of 

 silica, like springs of volcanic agency. The first or natural process can 

 have been but very slow, and so long, according to Professor Schimper, 

 that the wood could not have escaped total decomposition and destruc- 

 tion before its mineralization. In this assertion the celebrated professor 

 is certainly in error, for, from what we have said before, the woody 

 tissue, protected by immersion against the destroying influence of the oxy- 

 gen of air, is indeed indestructible for an indefinite length of time ; and be- 

 ing preserved for whole epochs in the soft state, can be thus slowly im- 

 pregnated by any kind of mineralizing elements which may be in solution 

 in the water. The opinion of Professor Schimper is still contradicted by 

 the fact that in this country silicified wood is found in our coal meas- 



