THE LAUEENTIAN 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- 

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

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

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

 reducing and solvent action of substances produced in the 

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

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

 borough, 200 feet thick,* must represent a corresponding 

 quantity of vegetable matter which has totally disappeared. 

 It may be added that similar demands on vegetable matter as 

 a deoxidizing 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 microscopic 

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

 modern sediments, algee have been converted into bituminous 

 matter, we cannot ordinarily obtain 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 bitumin- 

 ous 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 mountains 

 in Canada, the pores of brachiopodous shells and the cells of 

 corals have been penetrated by black bituminous matter, 

 forming what may be regarded as natural injections, some- 

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

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

 pact 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 

 * Geology of Canada, 18G3. 



