THE LAURENTIAN ROCKS. 31 



thoroughly disintegrated and bituminized before it was 

 changed into graphite. This would also give a probability 

 that the vegetation implied was aquatic, or at least that it 

 was accumulated under water. 



Dr. Hunt has, however, observed an indication of terres- 

 il vegetation, or at least of subaerial decay, in the great 

 ids of Laurentian iron ore. These, if formed in the same 

 inner as more modern deposits of this kind, would imply the 

 jducing and solvent action of substances produced in the 

 ly of plants. In this case such great ore beds as that of 

 [ull, on the Ottawa, seventy feet thick, or that near New- 

 >rougb, 200 feet thick,* must represent a corresponding 

 quantity of vegetable matter which has totally disappeared, 

 tt may be added that similar demands on vegetable matter as 

 deoxidizing agent are made by the beds and veins of metallic 

 sulphides of the Laurentian, though some of the latter are no 

 loubt of later date than the Laurentian rocks themselves. 

 " It would be very desirable to confirm such conclusions as 

 lose above deduced by the evidence of actual microscopic 

 itructure. It is to be observed, however, that when, in more 

 lodern sediments, algae have been converted into bituminous 

 itter, we cannot ordinarily obtain any structural evidence of 

 bhe origin of such bitumen, and in the graphitic slates and 

 lestones derived from the metamorphosis of such rocks no 

 )rganic structure remains. It is true that, in certain bitumin- 

 ms shales aiid limestones of the Silurian system, shreds of 

 )rganic tissue can sometimes be detected, and in some cases, 

 in the Lower Silurian limestone of the La Cloche mountains 

 Canada, the pores of brachiopodous shells and the cells of 

 )rals have been penetrated by black bituminous matter, 

 |Porming what may be regarded as natural injections, some- 

 times of much beauty. In correspondence with this, while in 

 jome Laurentian graphitic rocks, as, for instance, in the com- 

 )act graphite of Clarendon, the carbon presents a curdled 

 appearance due to segregation, and precisely similar to that of 

 the bitumen in more modem bituminous rocks, I can detect 

 the graphitic limestones occasional fibrous structures which 

 * Geology of Canada, 1863. 



