THE IRON-FORMATION ON BELCHER ISLANDS 435 
also be in the ferrous form and easily transported. It has generally 
been considered that practically all the silica in these pre-Cambrian 
iron-fermations has been carried in solution, and that very little 
of it has been clastic sediment. Experience with many different 
areas of these rocks shows that there is almost invariably a great 
deal of quartzite, arkose, or graywacke, distinct products of weather- 
ing, associated with the iron-formation, and that these often grade 
into the jaspers. The distinctly clastic sediments cease, and the 
cryptocrystalline forms of silica take their places. It scarcely 
seems reasonable that the deposition of clastic siliceous sediment 
should be so suddenly cut off in all cases and its place taken by 
chemical precipitates without a great deal of silt being deposited 
with the chemical precipitates. While this clastic material cannot 
now be identified in the jasper and chert, it seems probable that 
it is there, but indistinguishable because of metamorphism from the 
finely crystallized silica which makes up the bulk of all these jaspilite 
formations. 
Regarding the adequacy of the weathering processes to produce 
these deposits one has but to observe the great deposits of lateritic 
iron which have formed, and are continuing to form, in Cuba, 
India, and other warm countries to be convinced of the efficiency 
of. the weathering process. It is evident that the weathering of ~ 
iron-bearing rocks is almost constantly in operation, but it is owing 
to certain chemical and drainage conditions that the iron remains 
on the land as laterite and is not carried off to the sea or to other 
bodies of water. 
The chemical conditions depend upon two important factors, 
-one being the presence or absence of suitable solvents for the iron 
and the other the presence or absence of suitable precipitating 
agents which may throw the iron out of solution before it reaches 
the sea. That considerable iron which is left on the surface as a 
lateritic deposit and later washed to lower levels as a detrital deposit 
is carried in solution is evident from the fact that some of it takes 
on the concretionary form after being transported from its original 
location. 
From a consideration of the laterites the writer believes that 
the weathering of the basic igneous rocks would furnish plenty of 
iron to form the pre-Cambrian iron-formations, and that whether 
