1 2 FOSSIL MEDUSA. 



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 appearauce in polarized light, the extinction beiug 

 similar to that of cryptocrystalline 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 abseuce of the rhombohedral 

 cavities which appear to characterize most cherts that have formed as concretious 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 extinc- 

 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 uicols. 

 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 cases the nodules are clearly stratified; 



