ie? FOSSIL MEDUS#. 
Professor Iddings states his conclusions briefly in these words: 
The siliceous nodules consist of a mixture of granular quartz in allotriomorphic 
grains, occasional flakes of muscovite-mica, and a small amount of calcite in minute 
particles, numerous gas pores, and some coloring matter, probably carbonaceous. In 
one section there was a considerable amount of minute crystals, with very pronounced 
pleochroism, blue, purplish, and colorless, which is undoubtedly dumortierite, basic 
silicate of aluminum. 
Of the shale in which the nodule occurs, he says that a thin section 
shows very fine scales of mica in each micro-cryptocrystalline matrix, the 
only knowledge of which that we have is from chemical analysis. 
Dr. Hayes reported more at length, especially with relation to the 
question of the cherty character of the nodules: 
Thin sections of the nodules, when examined under the microscope with low 
powers, show a close resemblance to ordinary cherts. The similarity is particularly 
close in sections from those nodules which are free from coloring matter, as iron oxide. 
They have a finely mottled-gray appearance in polarized light, the extinction being 
similar to that of eryptocrystalline or chalcedonic silica. The chief difference between 
cherts and these nodules, when magnified less than 100 diameters, is the presence in 
the latter of more or less abundant mica scales, and the absence of the rhombohedral 
cavities which appear to characterize most cherts that have formed as concretions in 
a calcareous matrix. 
When high powers are used, 400 diameters and over, there is in every case a 
marked distinction between the true cherts and the nodules. The former show the 
same gray-mottled appearance in polarized light, while the latter are seen to be 
made up almost wholly of extremely fine grains with sharply defined outlines. These 
grains are of two kinds, although their form and size are very uniform. The first are 
colorless and have a low index of refraction, so that they are seen with difficulty in 
ordinary light. They come out prominently in polarized light, having sharp extine- 
tion and sometimes bright polarization colors. It seems quite probable that these are 
extremely fine grains of original detrital quartz. 
The second kind of grains have a high single refraction and contain some coloring 
matter, so that they stand out prominently in ordinary light. They have very little, 
if any, double refraction, so that they remain practically black between crossed nicols. 
These grains are probably a hydrous silicate of alumina—that is, clay. 
The color of the nodules is due chiefly to hydrated iron oxide. They contain also 
some carbonaceous matter, in extremely fine dust-like grains, and also some very small 
opaque cubes or octahedrons, probably pyrite or magnetite. They also contain more 
or less detrital mica scales, as mentioned above. If they contain amorphous silica, it 
is an inconsiderable amount. 
In most cases no concentric structure, due to true concretionary growth by the 
deposition of successive shells about a nucleus, appears either in polished sections of 
the nodules or under the microscope. In some eases the nodules are clearly stratified ; 
