402 E. O. Hovey — Cherts of Missouri. 



Lower Carboniferous. It is aside from the purpose of the 

 writer to discuss the geologic relations of the cherts, but the 

 Lower Magnesian cherts present some features which separate 

 them more or less sharply, lithologically, from the Lower Car- 

 boniferous ones. The former contain few fossils or moulds of 

 fossils, while the latter are usually crowded with stems and 

 plates of crinoids and other fossils, or the cavities left by their 

 removal, though occasionally a Lower Carboniferous chert is 

 found which shows no indication of organic remains. In both 

 groups the color varies very much from pure white to gray, 

 while very many of the cherts are stained brown or black by 

 iron, and one Lower Magnesian specimen is a decided pink. 

 The texture, likewise, is very various, some specimens being 

 very dense, aphanitic to the naked eye, and without fossils or 

 cavities, while others are vesicular from the solution of pebbles 

 or are full of cavities from the removal of fossils. There is 

 great difference, also, in the state of preservation of the chert, 

 much of it being almost perfectly fresh except for an outer 

 shell of decomposition, while other has suffered alteration 

 throughout its mass, as is shown in several Lower Magnesian 

 specimens and in the heavy beds of " tripoli " in the Lower 

 Carboniferous at Seneca and elsewhere. 



Petrography. — These cherts consist almost entirely of chal- 

 cedony or silica in the chalcedonic state, but quartz and opal 

 are present to some extent in some of the specimens. The 

 slides are almost colorless and featureless under the microscope 

 in ordinary light, but in polarized light the structure of the 

 rock is very clearly indicated and is shown to be a very fine 

 grained mosaic, mottled by reason of variation in fineness of 

 grain. In several of the specimens from the Lower Magnesian 

 series, notably one from Morgan County, the material is aggre- 

 gated into small spherules. These are optically negative, 

 which proves that they are made of chalcedony. Chalcedony 

 has a higher index of double refraction than quartz, but the 

 polarization colors in these sections rarely rise above gray of 

 the first order, because the grains, whether in mosaic or in 

 concentric spherule, are too small to give the thickness 

 required for the higher colors. For photo-micrographs show- 

 ing the mosaic and concretionary or oolitic structures of chert 

 the reader is referred to Irving and Yan Hise's treatise on 

 " The Penokee Iron- Bearing Series of Michigan and Wis- 

 consin," 10th Ann. Eep. IT. S. G-. S., 1890, Pt. 1, PI. 24, fig. 

 2, and PI. 28, fig. 2. The presence of opal silica is indicated 

 in some of the slides by apparently amorphous areas, but pos- 

 sibly with more definiteness in other specimens by the solu- 

 bility of a portion in KOH (caustic potash), though, as will be 

 shown, even this is not a certain criterion. Quartz occurs in 



