1918] Davis: The Radiolarian Cherts of the Franciscan Group 311 



The radiolaria are best preserved in the more opaque cherts, but 

 are not so well preserved in the more siliceous cherts, or in the clay- 

 stones. Microscopic examination reveals the fact that the radiolarian 

 claystones contain about as many radiolaria as the black chert. Yet 

 the silica content of the black chert is much higher, being about 

 ninety-one per cent as against sixty-eight per cent in the claystones. 

 The excess silica in the cherts is not believed to be of organic origin. 

 David and Pitman suggest that it may have been leached from the 

 tuffs and deposited in the chert horizons. 



David and Pitman believe that these claystones were deposited in 

 comparatively shallow water, because of the numerous plant remains. 

 Hinde believes that this is not certain proof and points to the dredg- 

 ings of Agassiz, which showed that plant remains might be car- 

 ried out for considerable distances from land and deposited in deep 

 water. Hinde is of the opinion that the claystones are the equivalents 

 of the recent Red Clays. The presence of ripple marks, however, 

 is certainly opposed to this interpretation. 



The Woolomin series, of lower Devonian age, extends from Tam- 

 worth to Bingara and appears to underlie the Tamworth series. Its 

 thickness is unknown. It covers an area of 1000 square miles. This 

 series comprises radiolarian claystones, tuffs, and spilitic lavas, feld- 

 spathic cherts with radiolaria, and red jaspers. Red, thin bedded, 

 radiolarian cherts form belts up to 100 feet thick in this series, though 

 the average thickness of the belts is considerably less. The whole 

 series has been intruded by serpentine. 



The jaspers contain radiolaria. They are hard, flinty rocks of a 

 bright red color, traversed by microscopic veins and stringers of 

 quartz. Hinde states that these rocks resemble those of California 

 and those from the Culm measures of Devon. 



At Jenolan Caves, west of Sydney, radiolarian rocks of Silurian 

 age appear. The series is described as consisting of red and green 

 claystones and talcose slates at the base. These are well bedded and 

 contain numerous radiolaria. Then comes 300 feet of rhyolite lava, 

 then 300 feet of claystones, then 500 feet of limestone and then an- 

 other 1000 feet of claystones and radiolarian cherts. 



Radiolarian cherts of Silurian age outcrop at several places in the 

 west part of New South Wales, where they are associated with rhyo- 

 lites and are very similar to the rocks of the Jenolan District. 



A radiolarian rock from Fanny Bay, Port Darwin, Australia, 

 described by Hinde, 48 appears to correspond to the radiolarian earths 



« Quar. Jour. Geol. Soc, vol. 49, p. 221, 1893. 



