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University of California Publications in Geology [Vol. ll 



shells. These show the perforations of the shell, the shell walls, and 

 the septa, perfectly preserved in the original fibrous aragonite. The 

 chambers in this particular specimen are not open but are filled with 

 crystalline calcite, though other specimens of the same sort show the 

 original shell walls with open chambers. 



The chert in the center of the specimen is also full of foraminiferal 

 shells. One or two lie partly in the chert, and partly in the shale, 

 occurring on the border between the two types of material. Under 

 the microscope the chert is found to consist of cryptocrystalline silica. 



Since the facts before presented bar the possibility of the cherts 

 being due to silicification of diatomaceous shales, one is forced to 

 adopt the other possibility, and believe that the white shale here is 

 the result of the partial disintegration of the chert. 



The black chert appears to consist of two varieties of silica — one 

 isotropic, the other anisotropic. The white shale contains only iso- 

 tropic silica and it would appear that the chalcedonic silica had been 

 removed by solutions, leaving the opaline silica behind in a porous 

 network. This is the reverse of what might be expected, but it may 

 well be that the hydrated form of silica is more stable under condi- 

 tions of weathering than the chalcedonic variety. Another peculiar 

 fact is that this change has taken place without the removal of the 

 lime carbonate of the shells. 



The extreme sharpness of the contact between the two types of 

 material might be suggested as opposed to this idea of partial leaching ; 

 neither would it be expected on the idea of silicification, so that it 

 cannot be raised as a serious objection. Moreover, as will be brought 

 out presently, this abrupt boundary seems to be usual when compact 

 silica of this sort is changed in the manner described. 



The partial removal of silica from massive, compact, siliceous 

 rocks, to form a porous material is a rather common occurrence. 



Keyes, 32 for example, has described an analogous occurrence in 

 certain cherts in Missouri. These cherts occur in small nodules and 

 in nodular bands in limestone. They are very compact, translucent 

 rocks when first removed from the limestone, breaking with a con- 

 ehoidal fracture. They quickly slacken under the air to a fine, in- 

 tensely white powder. Before being affected by atmospheric attack 

 these cherts do not appear to be fossiliferous. After the disintegra- 

 tion has gone part way they are found to be filled with fossils, whose 

 presence is revealed by the disintegration of the matrix. 



32 Keyes, C. E., A Remarkable Fauna at the Base of the Burlington Limestone 

 in Xortheastern Missouri, Am. Jour. Sei., vol. 144, p. 447, 1892. 



