512 » TRANSACTIONS OF THE CANADIAN INSTITUTE. [Vov. VIII. 
From this it is evident that while the action was in progress the calcite 
was in a sufficiently liquid state to completely dissolve away parts of the 
apatite crystal. 
54. Graphite is a mineral whose chemical composition most strongly 
suggests an organic origin. It is frequently found under conditions 
which point undeniably to this, as for instance where intrusive dykes cut 
across coal beds. ‘This mineral is, according to Van Hise °° never found 
in the Archean, its lowest horizon being the Algonkian which is in all 
probability a clastic series. It is interesting to note that in the amor- 
phous nodular phosphates of North Wales of Cambro-silurian age, graphite 
occurs in sufficient quantities to colour the rocks. 
55. Magnetite is a constant associate of apatite, often in very con- 
siderable amounts. In many places at the present time iron ores are in 
process of deposition, and the mode of formation can be readily traced. 
Water containing decaying vegetable matter, and in consequence strongly 
charged with humic acids, forms unstable soluble salts of iron, which are 
dissolved. ‘These, on exposure to the air, form hydroxides which grad- 
ually change to limonite, and this we know becomes magnetite on meta- 
morphism. According to Ehrenberg, the presence of diatoms has an equal 
part to play in the formation of iron ores. These microscopic plants 
have the power of concentrating the iron in water in their bodies which 
finally form deposits of great thickness. Thus either theory of the for- 
mation of the iron ores, leads us to the action of plant life. 
56. Anhydrite is another associated mineral whose presence has con- 
siderable bearing on the problem. It does not occur in very large quanti- 
ties, but that is to be expected from the great solubility of gypsum. La- 
goon muds are exceptionally rich in phosphoric acid and also in sulphur. 
The latter becomes oxidized to sulphuric acid which attacks the sur- 
rounding limestones at the same time that the phosphoric acid does and 
calcium sulphate and phosphate are formed.3! ‘This will explain the 
presence of anhydrite with apatite, for as a rule phosphates would be laid 
down in lakes or lagoons, and the ordinary way of the deposition of gypsum 
is in land locked bays of considerable depth, which gradually increase in 
concentration. 
57. Thus it is seen that four of the characteristic associated minerals 
which occur with apatite are those which under any other circumstances 
would be looked upon as of undoubted organic origin. 
30 Penrose. p, 80. 
31 Dieulafait, Ann. de Ch. et de Phys. 6 series 5: 204--1885. 
