116 PROCEEDINGS OF THE GEOLOGICAL SOCIETY.
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 Eozoon
penetrated by matter once bituminous, but now in the state of
graphite.
When palaeozoic land-plants have been converted into graphite, they sometimes perfectly retain their structure. Mineral charcoal, with structure, exists in the graphitic coal of Rhode Island. The fronds of ferns, with their minutest veins perfect, are preserved in the Devonian shales of St. John, in the state of graphite; and in the same formation there are trunks of Conifers (Dadoxylon onangondianum) in which the material of the cell-walls has been converted into graphite, while their cavities have been filled with calcareous spar and quartz, the finest structures being preserved quite as well as in comparatively unaltered specimens from the coal-formation*. No structures so perfect have as yet been detected in the Laurentian, though in the largest of the three graphitic beds at St. John there appear to be fibrous structures, which I believe may indicate the existence of land-plants. This graphite is composed of contorted and slickensided laminae, much like those of some bituminous shales and coarse coals; and in these there are occasional small pyritous masses which show hollow carbonaceous fibres, in some cases presenting obscure indications of lateral pores. I regard these indications, however, as uncertain; and it is not as yet fully ascertained that these beds at St. John are on the same geological horizon with the Lower Laurentian of Canada, though they certainly underlie the Primordial series of the Acadian group, and are separated from it by beds having the character of the Huronian.
There is thus no absolute impossibility that distinct organic tissues may be found in the Laurentian graphite, if formed from land-plants, more especially if any plants existed at that time having true woody or vascular tissues; but it cannot with certainty be affirmed that such tissues have been found. It is possible, however, that in the Laurentian period the vegetation of the land may have consisted wholly of cellular plants, as, for example, mosses and lichens; and if so, there would be comparatively little hope of the distinct preservation of their forms or tissues, or of our being able to distinguish the remains of land-plants from those of Algae.
We may sum up these facts and considerations in the following statements: — First, that somewhat obscure traces of organic structure can be detected in the Laurentian graphite; secondly, that the general arrangement and microscopic structure of the substance corresponds with that of the carbonaceous and bituminous matters in
- Acadian Geology, p. 535. In calcified specimens the structures remain in
the graphite after decalcification by an acid.
