320 



University of California Publications in Geology [Vol. 11 



The cherts are interstratified with shale in thin bands varying from 

 one-quarter of an inch to several inches in thickness. Thirty bands 

 were counted in one thickness of three feet. In some places layers of 

 chert pinch out, while other layers thicken into nodules. The chert 

 bands are generally traversed by a network of white quartz veins. 



The radiolarian remains are best seen on surfaces which have been 

 somewhat weathered; on these they stand out as rounded protuber- 

 ances. In many specimens it is possible to see the reticulated nature 

 of the skeletons with a hand lens. Some thin sections show the radio- 

 laria closely spaced and nearly in contact with one another ; in others 

 they are less numerous and more widely spaced. 



The shales which separate the chert bands vary in color from 

 brown or buff to nearly black. They have been searched for fossils, 

 but only one microscopic form, not identifiable, has ever been found. 



The pillow basalt which surrounds the chert shows relations that 

 suggest intrusion. It would appear to be analogous to some of the 

 intrusive basalts of the California Coast Ranges. Fox and Teall, how- 

 ever, believe that the igneous rock was a contemporaneous lava flow 

 because of the fine grain of the rock and the pillow structure which 

 it possesses. They suggest that it would be possible for lavas flowing 

 over the sea floor to tear up layers of the bottom deposits and insert 

 themselves between the layers. 



Radiolarian cherts are also known on the neighboring mainland. 62 

 They are dark to black rocks cut by numerous veins of quartz and 

 interbedded with clay slates. 



SOUTHEEN SCOTLAND 



Bedded radiolarian cherts form a conspicuous portion of the Ordo- 

 vician Rocks of southern Scotland. 03 They occur in both the Arenig 

 and the Llandeilo, the lowest and the intermediate divisions of the 

 Ordovician. 



The basal portion of the Arenig division comprises ellipsoidal 

 lavas, tuffs, and agglomerates of unknown thickness. Above these 

 volcanic rocks there are a few feet of black graptolite shale, and above 

 this are radiolarian cherts and interbedded mudstones. The Arenig 



«2 Hinde, G. .L, On Eadiolaria in Chert from Chypons Farm, Mullion Par- 

 ish (Cornwall), Quar. Jour. Geol. Soc, vol. 55, p. 214,'l899. 



63 Hinde, G. J., Notes on Eadiolaria from the Lower Paleozoic Eocks 

 (Llandeilo-Caradoc) of the South of Scotland, Ann. Mag. Nat. Hist., (6), 

 vol. 6, p. 40, 1890. 



Peach and Horne, The Silurian Eocks of Britain. I, Scotianu, Mem. Geol. 

 Surv. United Kingdom, 1899. 



