178 Th(i American GeologUt. September, i892 
which must be taken into account when tlie true explanation is 
discovered. These facts, which are based on observations made 
partly daring the present season, may be set down severally as 
postulates on which some satisfactory theory may possibly l>e. in 
part, built at some future time. 
1. The ore has a definite position in the stratification. 
li. Tt is underlain by a porous quartzyte. 
8. It is overlain by, or results from a change in a peculiar rock 
to which we have given the name taconyte. [To l)e described fully 
at a later date]. 
4. The whole ore bed is sometimes a breccia of some sedimen- 
tary rock,' lying loose, and sometimes compact brecciated or even 
conglomeritic phases are common. Rarely a pebbly ore is found. 
5. It is associated with much kaolinic, but water-stratified rock, 
and often the white kaolin, though embraced in the ore, is un- 
stained by it. 
n. It occurs at the same horizon the whole length of the Mesabi 
range. 
7. When the gabbro of the Mesabi range is adjacent this ore is 
found to be hard — either hematite or magnetite, but it is never 
affected by titanic acid — though it is by sulphur under such cir- 
cumstances. 
8. Apparently it runs southward with the dip of the formation, 
and by boring it is found under a large thickness of black slate 
about a mile south of the line of strike. 
9. The horizon of the ore is the same that has been assigned 
by the writer to the date of the disturbance of the gabbro flood 
But as that assignment is not sufficiently established it cannot be 
said that the ore has any relation of cause and effect to the gabbro. 
Concluition. 
There are but two items in the conclusion to which we are driven: 
First. The Mesabi ore is not satisfactorily explained by any 
theory that has yet been proposed for it, or for its equivalent 
(Gogebic) ore on the south side of the great lake. There are 
some facts that favor all of the theories that have been proposed, 
])ut they meet with opposing facts of greater import. 
Second. There is but one known cause acting with sufficient 
force and on a geographic area sufficiently wide, to which we 
can appeal for the geographic and stratigraphic distribution of 
