Review of Recent Geological Literal tire. 423 
contact metamorphism in limestones and calcitic schists. The forma- 
tion by pneumatolytic processes would conflict sharply with the char- 
acteristics mentioned under 2, 3, 5 and (i; and would not explain those 
tinder 1, 4, 7, and S. 
That our iron ores cannot be ascribed to metasomatic action (e. g., by 
the percolation of FeCo 3 solutions through limestone, by which 0aCO 8 
is dissolved and FeC0 3 deposited) is explained in detail in the original 
work. pp. 137, 138. 
The numerous different phenomena of these ores differ widely, it is 
true, but still the following series of general criteria can be deduced, to 
which a genetic Significance may be ascribed. 
1. The iron ore beds appear most frequently, although not always, 
toe-ether with or in the immediate vicinity of limestone or dolomite. In 
individual instances (Nordland, for example), it can be shown that the 
iron ore was formed at an earlier epoch than the accompanying: lime- 
stone or dolomite. 
2. In spite of this intimate association, the iron ores (magnetite and 
hematite) occur only sparingly in the limestone or dolomite, but are 
much more commonly mingled with quartz, or rather with acidic sili- 
cates. 
3. Mingled with the iron ores there is nearly always some manganese. 
while, on the other hand, Cu, Pb, Zn, Ni, Co, As. Sb, lb, etc. , occur only 
sporadically, or are entirely wanting; very little TiO a and Si; but oc- 
casionally some native "old. 
4. Apatite and other phosphatic minerals are present in varying 
amount, on the whole somewhat more abundant than in the neighbor- 
ing schists and limestones. 
Because of the intimate connection between our iron ores and the 
limestone and dolomite beds, and further because of the analogy be- 
tween the Archean and Cambrian magnetite and hematite beds, on the 
one hand; and the siderite, pyrolusite, and hematite deposits of the 
younger formations, besides the recenl lake and bog ores, on the other 
hand, the conclusion is reached that urn- ore* wereformed by precipitation 
from solutions of 'ferrous carbonate. We may state in advance that these 
carbonated solutions in earlier I imes. as at present, have arisen princi- 
pally through the destruction of rocks already in existence. IV 
CO 3 shows, as is well known, a strong tendency toward higher oxida- 
tion, for example, through the oxygen dissolved in water. The separa- 
tion of FeCO Can thus onlj lake place when oxidation is hindered or 
prevented by carbon ororganisms. That the iron was precipitated di- 
rectly as oxide, hydrous or anhydrous, in the Archean and Cambrian de- 
posits, in the same manner as in recent lake ores, is shown bj the fact 
that we often meet beds of iron ore wholly free from lime and magnesia, 
on the one hand, and of limestone and dolomite almost wholh free from 
iron, on the other, sharply distinct from each other and yel intimate!) 
intermingled, and these contain in most cases no trace of organic mat- 
ter which could have presented the oxidation. Only in rare instances 
corresponding to, the blackband ironstone (e. g. in Archean deposits of 
