CHERT FORMATIONS OF NOTRE DAME BAY 591 



In Notre Dame Bay the rock containing the most abundant 

 radiolaria is not a chert but is the red shale of the islands south of 

 Gull Island in Badger Bay. 



In other occurrences of radiolarian cherts it seems that the only 

 criterion employed for determining the abyssal origin was the pres- 

 ence of the radiolaria. 



Have the Radiolaria Played an Active Part in the Formation 

 OF THE Chert ? 



This question has often been raised in regard to the radiolarian 

 cherts. It was at first answered in the affirmative, but some of the 

 more recent investigators are inclined to the opposite opinion. 



Concerning Franciscan cherts Lawson says : 



As regards the organic origin of the silica of which the chert is composed, 

 it seems to the writer that there are features both in the slides and in the 

 field occurrence of the formation which do not harmonize with this supposition. 

 In the slides having the radiolarian remains, the latter generally occur as casts 

 of forms imbedded in a matrix of silica which shows no evidence whatever of 

 volcanic origin. The cavities of the radiolaria have been filled with chalcedonic 

 silica, and are in definite contrast with the nonchalcedonic matrix. The 

 discrete character of the fossils is significant of their mode of accumulation. 

 The silica seems to have been an amorphous chemical precipitate, forming in 

 the bottom of the ocean in which the radiolaria thrived. The dead radiolaria 

 dropped into this precipitate, became imbedded in it, and were so preserved.^ 



Davis also holds that the radiolaria of the Franciscan cherts 

 are merely incidental fossils which were imbedded in gelatinous 

 silica. 



In Notre Dame Bay some of the best preserved radiolaria occur 

 in jasper. In sections of this the radiolaria appear as clear areas, 

 sometimes showing an opaque center. The boundaries are often 

 exceedingly sharp. The areas are composed of quartz which is 

 relatively coarse-grained. Where radiolaria are observed in green 

 cherts some of the silica of the chert may be amorphous. In 

 neither case is there evidence that the rock in which the preserved 

 radiolaria are imbedded can be in any way made up of radiolarian 

 remains. 



'A. C. Lawson, "Sketch of the Geology of the San Francisco Peninsula," ijth 

 Ann. Kept., U.S. Geol. Surv. (1895), pp. 424-25. 



