330 University of California Publicatmis in Geology [ VoL - 11 



one horizon. The formation is extensive and radiolarian chert was 

 encountered at a great many points during the exploration. Molen- 

 graaff states that the Danau formation forms a belt at least sixty 

 kilometers wide and 650 kilometers long and covers an area of about 

 40,000 square kilometers. The cherts in the Danau formation are 

 estimated to be at least 100 meters thick. All over the area the radio- 

 larian rocks remain constant in their nature and show everywhere 

 the same macroscopic and microscopic characters. 



The cherts are described as being of variable color — purple, blue, 

 gray, red and white. Molengraaff distinguishes two types of rocks : 

 (1) radiolarian hornstone ; (2) argillaceous chert or siliceous clay- 

 shale. Both types of rock, hornstone and shale, pass gradually into 

 one another and all sorts of intermediate types occur. 



The radiolarian hornstones are semitransparent, hard, brittle 

 rocks with splintery fracture. They contain about ninety-seven per 

 cent silica and show numerous radiolaria. The siliceous clay-shales 

 are distinguished from the hornstones by their higher content of iron 

 and clay and by their smaller content of silica. The number of radi- 

 olaria in them is variable, but is always smaller than in the hornstones. 



The distinct stratification of the cherts of the Danau formation is 

 caused by thicker strata of pure radiolarian hornstone alternating 

 with thin layers which contain more clay and fewer radiolaria. 

 While the usual parting between the chert bands is argillaceous, 

 there are occasional partings that consist of limonite, which appears 

 to represent the alteration of originally interbedded iron ore. Occa- 

 sionally the cherts have been fractured and the resulting rock con- 

 sists of fragments of white or red chert cemented by brown veins of 

 limonite. 



Microscopically the cherts are seen to consist of cryptocrystalline 

 silica, and in this matrix the radiolaria are embedded. The radiolarian 

 tests are full of quartz which is comparatively coarse in its crystal- 

 lization. 



Radiolaria occur to a limited extent in the tuffs which occur in 

 close association with these cherts. 



Dr. Hinde examined the material collected on this expedition and 

 his report appears in an appendix to Molengraaff 's book. He states 

 that the radiolaria are very similar to those described by him from 

 the California Coast Ranges. As far as the radiolaria are concerned, 

 the cherts might be Lower Cretaceous, but he regards them as more 

 probably Jurassic for the reason that they are overlain unconformably 



