434 E. S. MOORE 
bodies of water, in which it seems probable that many of our pre- 
Cambrian deposits may have been laid down by processes of 
weathering. The largest known deposits of high-grade iron ore 
in the world, the Brazilian deposits, do not show direct relation to 
igneous rocks.t The great deposits of Lorraine, the Jurassic 
deposits of England, and our own Clinton ores show no direct 
relation to igneous eruptions. Going farther back into the pre- 
Cambrian rocks, it will be found that the greatest deposits of all, 
those of the Upper Huronian, show far less direct association with 
the basic igneous rocks than the smaller deposits of the Keewatin. 
This may be due to a large extent to the conditions of drainage, 
which must have been much better developed in the Huronian 
than in the Keewatin, if we can judge from the topographic features 
which are likely to have been produced during such a volcanic 
period as the Keewatin, and from the rocks which we now find 
making up the Keewatin series.?, There would be a tendency to 
deposit small and isolated bodies of iron-formation in the Keewatin, 
which later, on erosion, might add materially to the Huronian 
deposits. 
The problem of transportation of the silica and iron has always 
been a big one unless we invoke the aid of hot water and magmatic 
solutions. However, the work done on colloids in recent years has 
aided us materially toward a solution of this problem. It has been 
recognized by Lacroix that colloids are an important product in 
the weathering actions which produce laterites, and the authors of 
a recent paper on the origin of the Missouri cherts state that, so 
far as they know, silica is transported only in the colloidal form and 
not as a sodium silicate, since such a form dissociates to form 
colloidal silica. 
The silica set free from the decomposition of basic rocks would 
be almost entirely derived from the silicates, and it might be 
retained readily and carried in the colloidal form. The iron would 
« —. C. Harder and R. T. Chamberlin, “‘ Geology of Central Minas, Geras, Brazil,”’ 
Jour. of Geol., XXIII, 358-62, 385-404. 
2 In the recent volcanics on the island of Hawaii may be seen almost a complete 
imitation of the topographic features of certain uncovered Keewatin igneous areas. 
3G. H. Cox, R. S. Dean, and V. H. Gottschalk, Studies on the Origin of Missouri 
Cherts and Zinc Ores. Bull. 2, Vol. III, School of Mines and Metallurgy, University 
of Missouri. 
