1 66 The American Geologist. Marcii, i90o 
stromatoporoid coral and an orthoceras three or four inches 
long, also brachiopods and a favosite coral were seen. 
Taking into account its geologic position and lithologic 
character and relations, together with the lack of disturb- 
ances at this place it seems evident that the origin of the 
quartzyte was primaril)' the same as that of the limestones 
and shales beneath it. The presence of the fossils indicates 
its marine origin. The fossils, although but poorly preserved, 
are quite numerous and are of kinds having calcareous hard 
parts, but at present are siliceous. Along the septae of 
corals and cephalopods are minute cr)'stals of quartz due to 
action of percolating water. The former presence and 
power of the water is evidenced b}' the distribution of the 
iron which as stated above is mostl)- along seams, in fossil 
cavities and in fractures. 
In some localities limestones are interstratified with 
chert. This is especiall}' true of the Galena and Niagara in 
the lead region. Often half or more of the rock in beds of 
the latter is chert. The thick Niagara cap on the Platte 
mounds furnishes a good example of this, also the same 
stratum in many places in Jo Daviess county, Illinois; for 
example, the summit of Terrapin ridge southeast of the 
town of Elizabeth, where the fossils are silicified, and the 
Horseshoe mound southeast of Galena. This exceedingly 
cherty nature is onh* locally developed in the Niagara for 
there are quarries in it where chert is almost unknown. One 
such may be found in the southeastern corner of Derinda 
township of the county previously mentioned, and another 
in section 19, of Pleasant Valle}' township. 
Let us suppose that the limestone of the Niagara in the 
region of the Blue mounds was ver}- highly siliceous when 
deposited, i. e. that it contained much material from silica- 
secreting organisms as sponge spicules, and diatom tests 
mixed with the calcareous portion. This is legitimate be- 
cause certain regions in any sea are much richer in silica- 
secreting organisms than are others. This limestone up- 
raised later and subjected to the exceedingly long continued 
action of percolating waters, would result in the formation 
of a more siliceous rock if the lime were graduall}' removed. 
The siliceous shells were dissolved and the silica segregated; 
I 
