1880.] 477 [ Wadsworth. 
rests on those who hold that a sedimentary rock assumes the charac- 
,ters of an eruptive one. Hence we have here an unproved theory 
employed to prove another theory. 
2°. It requires that the highly refractory magnetite, hematite, and 
siliceous jaspilite, all of which it is denied could have been fused as 
an eruptive product, should have been rendered plastic and fluent, 
while the easily fusible argillites and chloritic schists as well as tal- 
cose, sericite, and other schists, sandstone and quartzite, neither be- 
came plastic, nor showed any signs of it. While rocks of both basic 
and acidic character, some even of nearly pure silica, are found in 
contact with ore and jaspilite which show eruptive relations, the 
former exhibit nothing of the kind. Surely selective metamorphism 
ean do no more than this! 
_So far as our microscopic examinations have gone the iron ore 
exists in octahedral crystals when crystallized, whatever may be its 
present state of oxidation. Also it is difficult to find in our collec- 
tion any specimens free from magnetic properties. It is to be remem- 
bered that Messrs. Brooks and Credner held that the ore was all 
originally magnetite, and that it in part had been subsequently 
changed to hematite. The microscope would seem to sustain their 
conclusion. It is to be remarked that the present magnetic state 
of the ore,appears, in some cases at least, to be directly dependent on 
the presence of later eruptive rocks. We found that at or near the 
contact of the ore with such rocks, it (the ore) was strongly mag- 
netic, but at a distance only slightly so. If the ore was originally 
all magnetite, it certainly was in the same condition in which it 
-abundantly occurs in various eruptive rocks; if originally hematite, 
it waS in the same state as it exists in less amount in some eruptive 
rocks, particularly acidic ones. So far as chemical objections exist to 
the presence of magnetite and hematite with siliceous minerals, we 
can sunply say that no eruptive rocks exist but the same objections 
lie against them. Since all lithologists are aware that magnetite and 
hematite occur in modern lavas, it seems probable that the difficulty 
rests with the present knowledge of the chemists and not with the 
lavas. 
We rest our conclusion that the jaspilite and iron ore in the Mar- 
quette district are eruptive upon the fact that they possess char- 
acters which eruptive rocks exhibit, especially in relation to other 
rocks, and which no sedimentary rock, proved to be such, has been 
known to have. They offer no characters inconsistent with those 
*@ 
