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University of California Publications in Geology [Vol. 11 



band of black shales. Indeed the cliff sections leave no room for doubt that the 

 radiolarian cherts were deposited contemporaneously with the volcanic eruptions, 

 for not only are they intercalated with the breccias but the latter likewise contain 

 fragments of organic chert with radiolaria, which must have solidified on the sea- 

 floor before its disruption by the explosion."-* . . . 



Here the intercalation of pyroclastic material, with radiolarian chert is admir- 

 ably seen in several alternations of these rocks. As each band of tuff contains 

 blocks of the chert, it is evident, that during pauses in the volcanic activity, the 

 radiolarian ooze consolidated on the sea-floor as chert, and was disrupted by the 

 successive explosions.65 . . . 



Where the cherts are associated with the non-fragmental pillow 

 lavas these are often found to contain included fragments of the 

 cherts of lower horizons. In rare instances radiolarian chert has 

 been observed completely filling the interspaces between the pillow- 

 form masses. 



The radiolarian cherts of this province are closely associated with 

 graptolite slates, often resting on or being overlain by them. Occa- 

 sionally thin layers of radiolarian cherts and their interlaminated 

 mudstones alternate with layers of graptolite shales. In the Upper 

 Llandeilo, black radiolarian cherts occur in bands, separated by 

 graptolite shales instead of by the ordinary shale partings. In a 

 few exceptional instances the cherts and associated mudstones are 

 interbanded with graywackes, and the alternations may be rather 

 rapid. 



In thin section, the light colored cherts are seen to be composed 

 of nearly clear cryptocrystalline silica somewhat clouded by very 

 minute mineral particles. In this matrix there are clear areas with 

 circular and elliptical outlines, representing the tests of radiolaria. 

 There is a notable contrast in the silica filling the tests, and that which 

 forms the matrix in which they rest. The silica of the matrix is much 

 finer in crystallization than that which fills the radiolarian skeleton. 



In the darker colored cherts, the matrix is stained by inclusions 

 of iron oxide, manganese, and carbon, and in these rocks the lattice 

 structures and spines of the radiolaria show distinctly as clearer 

 areas in the turbid ground mass. In the unstained cherts no structure 

 is observed in the remains of the radiolaria. 



Besides the radiolaria the cherts contain a few sponge spicules 

 with occasional remnants of uncertain origin. Occasionally the mud- 

 stones contain radiolaria, visible to the eye on fresh fractures. 



Near the contact with granitic rocks the radiolarian cherts are 

 metamorphosed. This metamorphism has occurred without change of 



64 Silurian Bocks of Britain. I, Scotland, p. 40. 

 es Ibid., p. 437. 



