The Columbian Exposition. — Williams. 345 
upon these operations, have produced all the various phases 
of the iron-bearing rock, including the ore-bodies. 
3. During the process of concentration, in areas of compar- 
atively free oxidation, iron replaces silica; in areas of ex- 
tremely scant oxidation, silica replaces iron. 
4. The great ore-deposits are believed to have formed in 
large areas of weakness, which have chiefly resulted from 
regional disturbances. The date of these disturbances, and 
of the beginning of the formation of some of the largest ore- 
deposits, is provisionally assigned to Keweenawan time. 
THE COLUMBIAN EXPOSITION: NOTES ON VARI- 
OUS EXHIBITS RELATIVE TO MINERALOGY 
AND PETROGRAPHY. 
By Geobge H. Williams, Baltimore, Md. 
There was at the Exposition a considerable amount of ma- 
terial of especial interest to students of mineralogy and pe- 
trography, but this was for the most part so widely separated 
and distributed through so many different buildings as parts 
of varied and little related exhibits, that a patient and pro- 
longed search was necessary to discover it. The following 
brief notes, which are the substance of information relative 
to these subjects communicated to the Madison meeting of 
the Geological Society of America, do not pretend to any 
completeness. They are merely jottings made in regard to 
apparatus and collections of mineralogical and petrographi- 
cal importance, as these happened, one after another, to come 
under the writer's notice in various parts of the Exposition 
grounds. 
I. Mineralogical Models ami Apparatus. 
One of the most noteworthy and novel, but at the same 
time probably one of the least noticed mineralogical exhibits 
was contained in a small case in the Russian pedagogical mu- 
seum, in the central west gallery of the Manufactures build- 
ing. It contained the kaleidoscopic mirrors for crystal] o- 
graphic demonstration, as suggested by Iless, and improved 
by Professor Federow of St. Petersburg (see Neues Jahrbuch 
fur Mineralogie, etc., 18N<>. J, p. 54; ib. 1890, I, p. 234; and 
the writer's P^lements of Crystallography, 8d edition, p. 196). 
