338 
MINING GEOLOGY 
of alkalies; but the solution of the iron would be more likely 
aided by the presence of acids. If carbonate minerals were the 
more abundant, as they certainly were in some of the other ranges, 
an alkaline bi-carbonate solution might have acted as a common 
solvent. Such solution obtains sometimes in connection with 
volcanic activities. In the present instance some alkali might 
have been derived from the great granite intrusion of the neigh¬ 
borhood. 
As to the primitive source of the metal-content of unique, 
thick, and extensive iron-bearing formations like those of the 
Lake Superior region Van Hise and Leith appeal directly and 
mainly to magnatic emanations from the contemporaneous basic 
igneous rocks. Repeated alternation of ore material in the beds 
as they now occur seems to be a significant fact bearing upon the 
history of the iron formation. There are hundreds of alternations 
of, fine magnetite and coarse fragrnental layers. Rhythmic sedi¬ 
mentation is manifestly due in some cases to a rhythmic supply of 
differing materials. In a broad way, the supply may have been 
ferruginous at one time, slaty at another, and cherty at another. 
If the materials, however, had been derived from volcanic sources, 
as sometimes suggested, it appea'rs improbable that supplies of 
iron oxide and silica could alternate so many times and on such 
a minute scale as the sediments indicate. It is unlikely that 
there could have been so many successive lava flows. Volcanic 
rhythms should produce alternations on a large scale. Further¬ 
more, climatic rhythms are also large features. Alternation of 
layers from one-tenth of an inch to six inches is better attributable 
to seasonal or other occasional changes in conditions. And these 
changes would affect a chemically depositing sediment only in 
shallow water. Since it is believed that the pebbles were formed 
in shallow water the inference is that standing water acted upon 
the ferruginous chert, enriching it much as the certain ores of the 
Lake Superior region are being enriched at the present day; by 
removal of the silica mainly, but only to small extent by the de¬ 
position of metal in its place. The suggestion here made as to 
the origin of the pure magnetite in no way conflicts with the 
possibility advanced by Van Hise and Leith, that some oxides 
may have been precipitated directly in very pure form. 
It can hardly be assumed without qualification, however, that 
