68 HISTORICAL PALAZONTOLOGY. 
tons of marine animals. It would, in fact, be a matter of 
great difficulty to account for the formation of these great cal- 
careous masses on any other hypothesis. (3) The occurrence of 
phosphate of lime in the Laurentian Rocks in great abundance, 
and sometimes in the form of irregular beds, may very possibly 
be connected with the former existence in the strata of the re- 
mains of marine animals of whose skeleton this mineral is a con- 
stituent. (4) The Laurentian Rocks contain a vast amount of 
carbon in the form of black-lead or graphite. ‘This mineral is 
especially abundant in the limestones, occurring in regular beds, 
in veins or strings, or disseminated through the body of the lime- 
stone in the shape of crystals, scales, or irregular masses. The 
amount of graphite in some parts of the Lower Laurentian is 
so great that it has been calculated as equal to the quantity of 
carbon present in an equal thickness of the Coal-measures. 
The general source of solid carbon in the crust of the earth 
is, however, plant-life ; and it seems impossible to account for 
the Laurentian graphite, except upon the supposition that it 
is metamorphosed vegetable matter. (5) Lastly, the great 
beds of iron-ore (peroxide and magnetic oxide) which occur 
in the Laurentian series interstratified with the other rocks, 
point with great probability to the action of vegetable life; 
since similar deposits in later formations can commonly be 
shown to have been formed by the deoxidising power of vege- 
table matter in a state of decay. 
In the words of Principal Dawson, ‘any one of these rea- 
sons might, in itself, be held insufficient to prove so great and, 
at first sight, unlikely a conclusion as that of the existence of 
abundant animal and vegetable life in the Laurentian; but the 
concurrence of the whole in a series of deposits unquestion- 
ably marine, forms a chain of evidence so powerful that it 
might command belief even if no fragment of any organic or 
living form or structure had ever been recognised in these an- 
cient rocks.” Of late years, however, there have been dis- 
covered in the Laurentian Rocks certain bodies which are 
believed to be truly the remains of animals, and of which by 
far the most important is the structure known under the now 
celebrated name of Zozodn. If truly organic, a very special 
and exceptional interest attaches itself to Hozodn, as being the 
most ancient fossil animal of which we have any knowledge ; 
but there are some who regard it really a peculiar form of 
mineral structure, and a severe, protracted, and still unfinished 
controversy has been carried on as to its nature. Into this 
controversy it is wholly unnecessary to enter here ; and it will 
be sufficient to briefly explain the structure of Hozodn, as eluci- 
dated by the elaborate and masterly investigations of Car- 
