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University of California Publications in Geology [Vol. 11 



a certain slight lamination, but does not show any trace of rhythmic 

 alternations of chert and shale. Evidently radiolarian oozes and 

 rocks formed from them do not show a bedding like the radiolarian 

 cherts. 



Occurrence of Chert in Lenslike Bodies. — The occurrence of iso- 

 lated bodies of chert, in the midst of the sandstone, is extremely diffi- 

 cult to reconcile with an idea that the cherts are radiolarian oozes. 

 If they are, no reason appears for the swarming of radio] aria in such 

 localized areas, save that of a local and limited supply of silica. The 

 only possibility which occurs here is to be found in some hypothesis 

 of siliceous springs. 



Conclusion. — In view of the various lines of evidence presented 

 and the peculiarities of the radiolarian cherts, it seems certain that 

 the cherts are not the equivalents of radiolarian oozes or of mixed 

 oozes of diatoms with radiolaria. One is forced to the conclusion that 

 a considerable part of the silica of the cherts has some other source 

 than the skeletons of radiolaria, probably being organic, while the 

 radiolaria are simply incidental fossils, caught in a deposit of silica 

 which was gelatinous at the time of their inclusion. 



Evidence that a Portion op the Silica of the Eadiolarian Cherts 

 was Chemically Precipitated 



It does not seem probable that the radiolarian cherts represent 

 pure radiolarian or mixed organic oozes containing radiolaria. The 

 occurrence of chert in lenses and nodules is more in agreement with 

 their interpretation as chemical precipitates of some sort. 



The general appearance of fresh, unaltered chert is in accord with 

 the idea that a considerable part of the silica was once in the gela- 

 tinous condition. The smooth, conchoidal fracture, waxy luster and 

 lack of grain are characteristic of substances which are believed to 

 be colloidal in origin. 



The fact that the cherts consist largely of chalcedony is also in 

 favor of the idea that they were originally in the condition of gela- 

 tinous silica. Lindgren 139 says : 



The work of Hein, Leitmeyer and others leads to the conclusion that chalce- 

 dony is in all cases composed of quartz fibers and that it always results from 

 the crystallization of gelatinous silica either "in statu nascendi" or at a later 

 time, and that gelatinous silica may in becoming crystalline either turn into 

 granular quartz or into fibrous quartz, i.e., chalcedony. 



139 Lindgren, W., Processes of Mineralizations and Enrichment in the Tintie 

 Mining District, Econ. Geol., vol. 10, p. 233, 1915. 



