The Iron Ores of the Mesabi Range. — Spurr. 337 
is in the vicinity. To the east of this upthrust area there is, 
as far as the Embarras lakes at least (a distance of five or 
six miles), a gentle folding of the strata, which appears t<> 
have been contemporaneous with the faulting. These dis- 
turbances may be provisionally believed to have occurred in 
later Keweenawan or post-Keweenawan time, and to have been 
contemporaneous with the monoclinal tilting. Figure '2, plate 
vni, is a section from west to east across these faulted and 
folded strata. 
Character of the Iron-bearing Hocks. 
The rocks of the iron-bearing member exhibit great di- 
versity. The most common sort is massive and siliceous, and 
is thickly spotted with small round darker areas, consisting 
mainly of iron oxide. From this there are many deviations : 
and often the different varieties cannot be said to resemble 
one another in any way. cither in the field or under the mi- 
croscope, or on chemical investigation. But specimens are 
constantly found which show one variety changing into an- 
other. Thus it soon became evident to the writer that all 
the rocks of the iron-bearing member, however different, were 
closely allied in origin, and were probably derived from a 
single primitive type. This same principle has already been 
shown by Irving and Van Ilise for the Penokce-( 1-ogebic rocks.* 
Nature of the Original Rock. 
A careful macroscopic and microscopic study of a large 
number of carefully selected specimens was made, with very 
satisfactory results. Nearly every detail of the changes from 
one phase into another could be made out and the causes 
assigned. The changes were seen to be those of nietasomato- 
sis, and the inciting agents seemed mainly atmospheric. The 
original rock of the series was quite unmistakable. Typically 
it may be briefly described as follows: 
In a ground-mass of cryptocrystalline, ehalcedonic, or finely 
phenocrystalline silica, are thickly strewn rounded or sub- 
angular bodies, made up chiefly of a green mineral, very 
slightly pleoehroic, and without cleavage: under crossed 
nicols it appears in some places amorphous, in others extin- 
guishing as an aggregate. 
*Tenlh Ann. Rep., !'. s. Geol. Survey . 
