20(i The American Geologiat. March, isns 
unhesitatingly assigns that of Pilot knob to a sedimentary origin, and 
the ore found in it to cotemporary sedimentary action. But this 
requires that it be of later date than the Iron mountain rock and 
unconformable upon it, a supposition fully borne out by the coarsely 
fragmental character of the rock of Pilot knob, and by the widely 
different structural character of the ore. The chief obstacle which 
occurs to us, against the infiltration hypothesis, is the fact men- 
tioned l)y ^Ir. Nason, that in the original vein ore are considerable 
crystals of apatite and occasionally of hornblende. 
It will be noticed that Mr. Nason's examinations of Pilot knob do 
not lead to the same result as those of Prof. Haworth, but he finds 
unmistakable evidence, admitted by Prof. Haworth, that the rock 
there furnishing the ore, while largely of porphyry conglomerate, is 
of sedimentary structure. Prof. Winslow says in his letter of trans- 
mittal: "During the past summer a conference and joint excursion 
to this field was arranged for. The party consisted of Prof. C. R. Van 
llise. Prof. "\Vm. B. Potter, Mr. Nason, Prof. Haworth and the writer. 
A result of this trip and the consequent discussion on the ground 
was the yielding, by Mr. Haworth, of his position with reference to 
the origin of the porphyry conglomerates, iron ores and other im- 
mediately associated beds occurring at the summit of Pilot knob and . 
at a few other localities in the immediate vicinity. Mr. Haworth 
now concedes that the evidence favors the conclusion that these 
beds are of sedimentary origin rather than of igneous origin as pre- 
viously advocated by him." (See American Geologist, vol. i, pp 280 
and 363,1888, also vol. ix, p. 55.) Mr. Haworth's earlier determi- 
nation was based on microscopic examinations of rock samples, after 
brief study in the field ; Mr. Nason's on field studies solely, though he 
had Mr. Haworth's publications constantly in mind, and was com- 
pelled by them to give the subject more thoughtful and thorough at- 
tention. The result, as it appears now, is another striking instance 
of the pitfalls into which petrographical students are liable to lead 
the unwary geologist. It shows either the incompetency of their 
criteria or of their observations, and the superiority of field studies 
conducted by a competent geologist. 
The red hematites of the Lower Carboniferous are c|uite unimpor- 
tant, but they constitute original and distinct beds among the sedi- 
mentary strata. 
The specular ores of the sandstone region, one of the strata of the 
"Ozark uplift," called Roubidoux sandstone by Mr. Nason, are fine- 
grained and compact, and apt to be of low grade by reason of large 
per-centages of silica. They are supposed to be due to concentra- 
tion by percolating water from other and higher strata which have 
been removed subsequently by disintegration and erosion. 
The limonites are principally confined to the Ozark district, but not 
entirely to the Ozark series. They affect the limestones, but also occur 
in cherty clay, the residuary product of the limestone. They take on 
all the pseudomorphous forms peculiar to limonite, and are found in 
