40 THE GEOLOGICAL HISTORY OF PLANTS. 



shales and limestones of the Silurian owe their carbona- 

 ceous matters to the decomposition of Algae, though pos- 

 sibly some of it may have been derived from Graptolites 

 and other corneous Zoophytes. In any case, such micro- 



Fig. 14. — Silurian vegetation restored. Pr ot annular ia, Berwynia, Nema- 

 tophyton, SpJienophyllum, Arthrostigma, Psilophyton. 



scopic examinations of these shales as I have made, have 

 not produced any evidence of the existence of plants of 

 higher grade, while those of the Erian and Carboniferous 

 periods, similar to the naked eye, abound in such evi- 

 dence. It is also to be observed that, on the surfaces of 



