DAWSON——LAURENTIAN GRAPHITE. 115 
mode of occurrence to the bituminous matter in bituminous shales 
and limestones. 
We may compare the disseminated graphite to that which we 
find in those districts of Canada in which Silurian and Devonian 
bituminous shales and limestones have been metamorphosed and 
converted into graphitic rocks not dissimilar to those in the less 
altered portions of the Laurentian*. In like manner it seems pro- 
bable that the numerous reticulating veins of graphite may have 
been formed by the segregation of bituminous matter into fissures 
and planes of least resistance, in the manner in which such veins 
occur in modern bituminous limestones and shales. Such bitumi- 
nous veins occur in the Lower Carboniferous limestone and shale of 
Dorchester and Hillsborough, New Brunswick, with an arrangement 
very similar to that of the veins of graphite; and in the Quebec 
rocks of Point Levi, veins attaining to a thickness of more than a 
foot, are filled with a coaly matter having a transverse columnar 
structure and regarded by Logan and Hunt as an altered bitumen. 
These palozoic analogies would lead us to infer that the larger 
part of the Laurentian graphite falls under the second class of 
deposits above mentioned, and that, if of vegetable origin, the organic 
matter must have been thoroughly disintegrated and bituminized 
before it was changed into graphite. This would also give a pro- 
bability that the vegetation implied was aquatic, or at least that it 
was accumulated under water. 
Dr. Hunt has, however, observed an indication of terrestrial vege- 
tation, 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, 70 feet thick, or that 
near Newborough, 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, 
Algze have been converted into bituminous matter, we cannot ordi- 
narily obtain any structural evidence of the origin of such bitumen, 
and in the graphitic slates and limestones derived from the metamor- 
phosis 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 moun- 
tains in Canada, the pores of brachiopodous shells and the cells of 
corals haye been penetrated by black bituminous matter, forming 
* Granby, Melbourne, Owl’s Head, &c., ‘Geology of Canada,’ 1863, p. 599. 
t Geology of Canada, 1863. 9 
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