1918J Davis: The Radiolarian Cherts of the Franciscan Group 333 



In thin section this rock is seen to consist of a fine-grained, amorphous ground- 

 mass of ehalcedonic silica, copiously stained with oxide of iron, with almost in- 

 numerable round and oval areas which are more or less clear. . . . 



Between crossed nicols these areas are seen to be filled with a doubly refract- 

 ing material which often exhibits undulatory extinction, and which is in a more 

 or less granulated condition ; by using a higher power, it is clearly evident that this 

 granulated material, with every optical character of ehalcedonic silica, constitutes 

 both the ground mass and the clear areas. 



Fanning reports that the andesite below the cherts of Pangasinan 

 has been extensively silicified and believes that Lawson's theory of 

 siliceous springs is applicable here, the underlying formation being 

 silicified by waters which rose through it. 



Associated with these cherts there are schists of various kinds, 

 with serpentines. Also a peculiar "eruptive conglomerate" consist- 

 ing of rounded masses of lava embedded in a soft matrix. 



Smith regards the cherts of the Philippines as contemporaneous 

 with the radiolarian cherts of Borneo, Java, and the Dutch East- 

 Indian Archipelago. 87 



The Novaculites op Arkansas, Oklahoma, and Texas 88 



These siliceous rocks are somewhat different from typical radio- 

 larian cherts in that they are usually rather more massive and the 

 regular alternation with shales is not so perfectly developed. Gris- 

 wold thus describes the novaculite of Arkansas : 



The Arkansas novaculite resembles chert not only in structure and composi- 

 tion but also in the manner of occurrence. Both occur as stratified rocks, but 

 there are some general differences. The novaculites are much more homogenous 

 than cherts. Chert beds are commonly associated in a nodular form with lime- 

 stone strata; the novaculites on the other hand, occur in massive strata, usually 

 presenting plane surfaces and having only thin layers of shale interbedded. Five 

 or six hundred feet is the common thickness of the novaculite formation, which 

 generally includes some flinty shales and soft shales or sandstones. The novacu- 

 lites proper are the prominent members of the formation, however, and occur in 

 massive beds from a few inches to twelve or fifteen feet in thickness. When 

 thinner than about four inches the beds generally lose their novaculite character 

 and are more like flinty shale. The massive beds are so closely associated that 

 there often appears to be no parting between them, but stratification lines are 

 indicated in quarries by thin seams of clay. 



8 7 Smith, W. D., Notes on Badiolarian Cherts in Oregon, Am. Jour. Sei 

 vol. 42, p. 299, 1916. 



88 Griswold, L. S., Whetstones and the Novaculites of Arkansas, Ann. Bep. 

 Geol. Surv. Arkansas, vol. 3, 1890. 



Derby and Brainier, On the origin of certain siliceous rocks, Jour. Geol., 

 vol. 6, p. 366, 1898. 



