560 RADIOLARIA IN PALAEOZOIC ROCKS, 



In some of the harder and more cherty beds very minute bodies 

 like those in the Pre-Cambrian phthanitic quartzite of Brittany 

 are noticeable, "006 to -013 mm. There is no evidence to show 

 tliat these are oi'ganic. 



Under favourable conditions of light the latticed structure of 

 the radiolarian shells can be distinctly seen in the coarse material 

 resulting from the disintegration of the soft shales in water. 



A few minute dentated plates, perhaps radulpe of gasteropods, 

 of dark brownish tinge are associated with the radiolaria. Detrital 

 fragments, except mica flakes, are either wholly wanting or 

 extremely minute, '03 to "065 mm. in diameter. 



Rarely limestone is associtited with the radiolarian rock, and in 

 the limestone are casts of radiolaria in calcite and also of sponge 

 spicules. Entomostraca, crinoids, and Endothyra contribute to 

 form limestones near this radiolarian horizon. 



In the majority of the Culm siliceous rocks the radiolaria are 

 now in the condition of solid casts of the original forms; their 

 skeletal walls have entirely disappeared, and the individual casts 

 are only bounded by the siliceous matrix of the rock, and are 

 without definite even outlines. In such instances only the size and 

 general foi'm with the radial spines can be distinguished. 



In some cases the tests have been naturally stained a brown or 

 amber tint, and in such cases the latticed character of the shell 

 is quite visible. 



Mr. Fox in a later paper* thus summarizes the evidence : — • 

 "These radiolarian rocks of Cornwall may be compared with 

 similar rocks of S. Scotland and with those descril^ed by Rust 

 from the Hartz, as well as those from the coast ranges of Cali- 

 fornia, of Jurassic age or older. . . . It is evident from 

 these examjDles that in the process of the formation of cliert the 

 finer structures and the more delicate forms of the microscopic 

 organisms disapj^ear nearly entirely, so that it is but rarely that 

 traces of them are now to be seen in the older cherts." 



* "The Radiolarian Cherts of Cornwall." Trans. Roy. (!eol. Soc. Corn, 

 read Nov. 8, 1895. 



