14 THE GEOLOGICAL HISTORY OF PLANTS. 



feet thick, or that near Newborough, two hundred feet 

 thick,* must represent a corresponding quantity of vege- 

 table matter which has totally disappeared. It may be 

 added that similar demands on vegetable matter as a 

 deoxidising agent are made by the beds and veins of 

 metallic sulphides of the Laurentian, though some of 

 the latter are no doubt of later date than the Laurentian 

 rocks themselves. 



It would be very desirable to confirm such conclusions 

 as those above deduced by the evidence of actual micro- 

 scopic structure. It is to be observed, however, that 

 when, in more modern sediments, Algae have been con- 

 verted into bituminous matter, we cannot ordinarily ob- 

 tain any structural evidence of the origin of such bitumen, 

 and in the graphitic slates and limestones derived from 

 the metamorphosis of such rocks no organic structure 

 [remains. It is true that, in certain bituminous shales 

 and limestones of the Silurian system, shreds of organic 

 tissue can sometimes be detected, and in some cases, as 

 in the Lower Silurian limestone of the La Cloche Mount- 

 ains in Canada, the pores of brachiopodous shells and 

 the cells of corals have been penetrated by black bitu- 

 minous matter, forming what may be regarded as natural 

 injections, sometimes of much beauty. In correspondence 

 with this, while in some Laurentian graphitic rocks, as, 

 for instance, in the compact graphite of Clarendon, the 

 carbon presents a curdled appearance due to segregation, 

 and precisely similar to that of the bitumen in more 

 modern bituminous rocks, I can detect in the graphitic 

 limestones occasional fibrous structures which may be 

 remains of plants, and in some specimens vermicular 

 lines, which I believe to be tubes of JEozoon penetrated 

 by matter once bituminous, but now in the state of 

 graphite. 



* " Geology of Canada," 1863. 



