1918] Davis: The Bacliolarian Cherts of the Franciscan Group 299 



ALTEEATION OF CHEET 



The peculiar relations between black flinty chert and white shales 

 seen in certain instances require some discussion. Fairbanks 31 makes 

 the following' statement : 



Some portions of the diatomaceous beds have undergone a transformation and 

 hand specimens were obtained in which the flinty alteration product appeared 

 sharply marked off from the unaltered portion by lines running directly across 

 the bedding. 



One occasionally sees masses of black flinty chert which show irreg- 

 ular patches of porous white shale within them, the boundaries be- 

 tween the two lithologic types being quite sharp and distinct. 



One specimen in the possession of the writer brings out these rela- 

 tions very well ; it is only one of many similar occurrences. It consists 

 in its central portion, of typical flinty chert, black in color. Toward 

 the borders, the chert becomes slightly discolored and is brownish 

 instead of black. The boundaries between the black chert and the 

 surrounding brownish chert are vague and irregular (plate 35c). 



The outer portions of the mass, and a portion of the central part, 

 consist of white shale. This material is firm and resists crushing to a 

 considerable degree, though it is porous and rather easily scratched 

 with a knife. The boundary between the brown chert and the white 

 shale is irregular, there being numerous indentations and reentrants 

 in the boundary line ; yet the actual line of separation between these 

 two lithologic types is sharply defined. One passes immediately from 

 chert to shale without a transition zone. There are, however, occa- 

 sional small elliptical patches of brown chert within the white shale. 

 The boundary between the white shale and the chert bears no relation 

 to bedding, so the relation between the two materials was not pro- 

 duced in the original deposition of the rock. 



Two possibilities present themselves in explanation of the observed 

 relations : either the black chert is due to the introduction of silica 

 into the shale ; or the white shale is the result of the partial disintegra- 

 tion of the chert — the removal of a portion of its silica, leaving the 

 remainder behind in a porous condition. 



In thin section the white shale around the outer portion of the 

 specimen is found to consist almost entirely of amorphous silica. 

 Scattered through this material are numerous grains of polarizing 

 minerals, quartz and feldspar being determinable. No diatoms or 

 radiolarians appear, but the section shows numerous foraminiferal 



3i San Luis Folio, op. cit. 



