1918] Davis: The Radiolarian Cherts of the Franciscan Group 329 



lavas conditions were set up in the ocean water that prevented the 

 existence of those organisms which devour the radiolaria. With their 

 enemies thus removed the radiolaria were able to reach an enormous 

 development. 



Lotti objects to the interpretation of the jaspers as radiolarian 

 oozes. He points out that many of them show no radiolaria in thin 

 section. He believes that most of the difficulties would be removed if 

 the jaspers were regarded as muds, silicified soon after their deposi- 

 tion on the sea floor, as an after-phase of the eruption of the igneous 

 rocks. Such conditions would produce also an increase in the num- 

 ber of radiolaria. 



More recently C. S. Du Riche Preller 82 has suggested that these 



rocks are silicified limestones: 



Associated rocks are the semi-crystalline schists known as ftaniti and diaspri, 

 viz., silico-ealcareous, reddish and green schists, harder than limestone indurated 

 by taking up silica at the expense of lime, and containing radiolaria. Both, and 

 notably the more highly indurated diaspri, form bands on the margins of ophio- 

 litic rocks in proximity to calcareous masses. It is a noteworthy feature that 

 metalliferous deposits are found only in euphodite and diabase, never in serpen- 

 tine, though often near the contact of the latter ; again, manganese occurs not in 

 the ophiolitic rocks proper, but in the diaspri masses, though in the vicinity of 

 the former. 



BORNEO 



Molengraaff 83 has described radiolarian cherts from Borneo which 

 resemble those of California in many respects. The radiolarian cherts 

 in this region form part of the Danau formation, which comprises clay 

 slates, quartzites, sandstones, cherts, jaspers, some small lenses of lime- 

 stone, hornstones, diabase tuffs, tuff breccias, diabase, diabase porphy- 

 rite, serpentines, gabbros and norites, together with some amphibolites 

 and glaucophane schists. Some of the diabase occurs in the form of 

 dykes and intrusive sheets, while part of it appears to have been ex- 

 trusive. The serpentines and gabbros are intrusive into the sediments 

 and the amphibolitic schists appear to be related to the contacts of 

 these basic intrusives. 



The Danau formation has been considerably folded and disturbed 

 so that the cherts dip at steep angles and show contortion and shear- 

 ing to a notable degree. The disturbance has been so great that it is 

 not possible to be certain whether or not the chert occurs at more than 



82 The Ophiolitic Groups of the Ligurian Apennines, Geol. Mag., Decade vi 

 vol. 3, p. 489, 1916. 



83 Molengraaff, G. A. F., Geological Explorations in Central Borneo. Amster- 

 dam, 1902. 



Ibid., On Oceanic Deep-Sea Deposits of Central Borneo, Kon. Akad. v. Weten- 

 schappen te Amsterdam, Proc. Sci. Sec, vol. 12, p. 141, 1909. 



