1880.] , AT5 [ Wadsworth, 
It seems that many geologists and most chemists use this course of 
reasoning regarding all eruptive rocks, except those of recent forma- 
tion or those whose origin is immediately obvious. Why not study 
thoroughly in each case, the internal structure and the relations of a 
rock to its adjacent rocks before deciding upon its origin? 
6°. ‘The presence of phosphoric acid could only have been taken 
as proof of sedimentary origin by those who had no knowledge of 
eruptive rocks, since it is well known to occur in many of the latter. 
This has been shown both by chemical and microscopic analysis. 
Even if this were not the case, the presence of phosphoric acid could 
not be made proof of the organic origin of the ores of the Marquette 
district, since they are exceptionally free from phosphoric acid. 
We have now taken up all the evidence which we are aware has 
been used to prove the sedimentary origin of the jaspilite and ore. 
The characters used as proof seem to be such as are common to both 
sedimentary and eruptive rocks or are of no weight. 
It does not appear that those who advocate the sedimentary origin 
of the ore have ever made any careful study of its relations, but have 
rested their claim principally on the “ striped”’ appearance of the 
rock and the contortion of the stripes. Of the more recent authors 
who advocate the sedimentary origin, it does not appear that Dana, 
Hunt, Lesley, Newberry, and Winchell have ever studied the rocks 
in place, or, so far as their publications show, have ever been on the 
ground. Of the others, Brooks, Credner, Kimball, and Wright, all 
except Credner appear to be mining engineers, who at the time of 
their examination of the region gave no evidence that they had made 
a special study of the history and origin of rocks, or were even strati- 
graphical geologists. We claim that questions of the kind above dis- 
cussed belong to the domain of the petrographer, one who is con- 
versant with both petrology and lithology, and are not in the prov- 
ince of the chemist, mineralogist, stratigraphical geologist, palaeon- 
tologist, mining engineer, or lithologist as such. 
It now remains to give the evidence in behalf of the eruptive ori- 
gin of the jaspilite and ore. 
The prominent fact to be urged on this side is that wherever the 
contact of these rocks with the country rock could be studied, that 
contact was always an eruptive one. ‘The phenomena of the contact 
of eruptive rocks with other prior existing rocks are well known and 
not easily mistaken by one familiar with their study, however much 
they may trouble the chemist, mineralogist, stratigraphical geolo- 
gist, palaeontologist, mining engineer or lithologist. 
