588 EDWARD SAMPSON 



Graham. This series which may be about 500 feet thick contains 

 occasional beds of tuff, tuffaceous conglomerates, and breccias. 

 There are in the section beds of massive black chert which may be 

 interbedded with the tuffs. 



The radiolarian cherts of the Franciscan Group are unique in 

 some respects. They have been fully described by Davis.^ The 

 Franciscan Group over about 1,000 miles of the Coast Ranges of 

 CaUfornia and Oregon and in the Olympic Mountains of 

 Washington. The group shows great lithological uniformity 

 throughout. It consists of a thick series of arkose sandstones, 

 radiolarian cherts, and foraminiferal limestones, with subordinate 

 shale and conglomerate. Pillow basalts and diabases are common. 

 Some of them are believed to be intrusive. In the vicinity of 

 San Francisco Bay there are two heavy formations of chert of 530 

 and 900 feet in thickness. Besides these persistent formations there 

 are many small isolated lenses of chert, some of them outcropping 

 over several acres. The bedding in even the thickest of these 

 formations is exceedingly irregular, individual beds showing 

 pinchings, swelhngs, and lens-Hke forms which are compensated 

 for by the adjacent beds. 



The radiolarian cherts of the British Isles are particularly 

 interesting in connection with the present investigation. These 

 cherts not only have an association almost identical to that of 

 many of the Notre Dame Bay cherts, but some of them are con- 

 temporaneous. Moreover where fossils have been found in Notre 

 Dame Bay they are of types so nearly identical with those of Great 

 Britain as to imply a connection or some means of migration between 

 the two regions. 



On Mullion Island,^ off the southwest coast of Cornwall, occur 

 radiolarian cherts which are probably of Ordovician age, perhaps 

 Lower Ordovician. Chert is here closely associated with pillow 

 basalt and is found in a series of shales and limestones which are 

 underlain and overlain by igneous rocks. The chert is inter- 

 stratified with shale and occurs in bands which vary in thickness 



'E. F. Davis, op. ciU, pp. 235-432. 



"H. Fox and J. J. H. Teall, "On a Radiolarian Chert from Mullion Island," 

 Quar. Jour. Geol. Sac, Vol. XLIX (1893), pp. 211-20. 



