336 



University of California Publications in Geology [Vol. 11 



Under the microscope the chert exhibits circular or nearly circular aggregations of 

 coarser-grained quartz particles with wavy extinction in a fine and uniform- 

 grained matrix of interlocking quartz granules. Each circular aggregation of 

 coarser quartz particles is a well-preserved radiolarian skeleton. Eadiolarian 

 remains make up from five to fifty per cent of the total mass of the Santiago 

 chert. This formation is regarded by Ulrich as most likely Kinderhook (lower 

 Mississippian) in age. 



Whatever may be the origin of the novaculites, the Santiago chert appears to 

 be either a truly pelagic deposit or to have been deposited under conditions more 

 or less unique and of which there is no known counterpart at the present day. 

 Shallow marine waters receiving absolutely no terrigenous sediment and deposit- 

 ing no limestone are indeed conceivable but it is extremely likely that such shallow 

 water deposits would contain remains of other forms of life in addition to 

 radiolaria. 



I may add that in all the slides examined by myself the only ones which con- 

 tained the rhombohedral-shaped cavities which Griswold thought were once occu- 

 pied by calcite crystals were of the more porous variety of the novaculite known 

 as the Washita stone. 



In the field, in the hand specimen and under the microscope I found nothing 

 that I could consider as doing anything but support Branner 's view that the 

 novaculites are simply metamorphosed cherts. Lawson 's view that the metamor- 

 phism of the Franciscan cherts is a change from the amorphous soluble to the 

 crystalline insoluble variety of silica somewhat analogous to the process of devitri- 

 fication of lavas may apply with equal force in the case of the novaculites and 

 Santiago chert. 



OCCURRENCES OF BEDDED CHERTS AND JASPERS 

 WITHOUT RADIOLARIA 



Besides the bedded cherts which contain radiolaria, occurrences 

 of bedded cherts are known which are similar, in most respects, to the 

 radiolarian cherts, except that the skeletons of radiolaria are absent. 



Cherts and Jaspers of the Lake Superior District 



Examples of bedded cherts and jaspers which do not contain 

 radiolaria are to be met with in the various iron formations around 

 Lake Superior. These iron formations are found at various horizons 

 in the pre-Cambrian rocks of that region, and in spite of the fact that 

 they occur at different horizons, and are in many instances separated 

 from one another by great unconformities, they show a remarkable 

 similarity in their lithologic characters. Van Hise and Leith 92 thus 

 describe the iron formations, pointing out the distinction between 

 an ' ' iron ore ' ' and an ' ' iron formation ' ' : 



«2 Geology of the Lake Superior Region, U. S. Geol. Surv. Monograph 52, 

 pp. 461-462, 1911. 



