Lake Superior Iron Ores.—S purr. 347 
“A rock exhibiting well the transition between the glauconitic 
limestone and the jasperoid was collected on the Yukon river in the 
lower Ramparts. Here in a cliff was seen a thin seam of bright red 
jasper, with dark-green fine-grained rocks on both sides, which was 
in immediate contact with coarser-textured green rock shown by 
microscopic examination to be typical tuff. A thin section of the 
rock at the contact of the bright-red jasper seam with the dark-green 
chert is made up mostly of cryptocrystalline silica, which is chalce- 
donic in places, with spherulitic aggregates showing dark crosses un- 
der crossed nicols. Glauconite is abundant in irregular grains of 
- all sizes, and the decomposition of this mineral, forming chalcedonic 
silica and iron oxide, is seen in all its stages. The process is that 
observed by the writer in the rocks of the Mesabi iron range. This 
decomposition accounts for the ragged outlines of the grains of 
glauconite. The iron is dark red, apparently of ocherous hematite, and 
occurs everywhere, though it is considerably less in amount than the 
glauconite. It shows a tendency to accumulate in irregular clumps. 
Calcite in frequent ragged areas is residual, being encroached on by 
cryptocrystalline silica, which is evidently replacing it. * * re 
“In this thin section are seen organic remains of complicated 
structure. The structure is brought into prominence by the increased 
amount of glauconite and iron oxide which have formed in the canals 
and other cavities. Mr. Bashford Dean, of Columbia College, has 
examined this section and has determined the structure as unquestion- 
ably that of a fish-tooth. 
“A section of the bright-red jasper into which the green rock 
passes, taken only a few inches from the specimen above described, 
is composed entirely of very fine-grained silica, which is stained 
throughout with iron oxide so deeply that the rock is aphanitic even 
under the microscope. 
“Green and red jasperoids having the same structure as above 
described, with the exception of the residual glauconite and calcite, 
are frequent in the rocks of the Rampart series, and by reason of their 
superior hardness are most conspicuous in conglomerates which have 
been derived from these rocks.” 
In Texas, the glauconite, in the Tertiary glauconite sands, 
has, according to Penrose and other writers,* furnished, by its 
decomposition, limonite iron ores in large amount. 
The Origin of Glauconite. 
Dana? observes that the kinds of glauconite are: 1. Green 
earth of cavities in eruptive rocks. 2. Green grains of sand 
beds of rocks, as of the green sand of the chalk formation. 
The second kind is of course the most important, but the 
*Resumé by J. F. Kemp, Ore Deposits of the United States and Canada. 
Fourth edition, p. 98. 
7 System of Mineralogy, 6th ed., p. 683. 
