1918] Davis: The Radiolarian Cherts of the Franciscan Group 387 



Variation Due to Regular Seasonal Changes 



It might be possible to explain the rhythmic bedding as due to a 

 regular alternation of river currents with seasonal variations. 



OTHEE EXAMPLES OF RHYTHMIC ALTERNATION 



Explanations of this type have been advanced to explain similar 

 rhythms in stratification in other instances. A case in point is the 

 laminated glacial clay. This clay occurs in thin beds, which alternate 

 with layers of lime carbonate, or there may be alternations of clays 

 of different colors, e.g., red with gray, or alternations of clay with 

 sandstone. The thickness of the beds is very uniform and is rather 

 small, ranging from one-half inch to one and one-half inches. These 

 banded clays have been described from the Pleistocene glacial deposits 

 at several points in the northern United States 152 and in Canada. 153 

 Similar laminated clays in Sweden have been described by Gerard 

 de Geer. 154 Recently Sayles 155 has described laminated argillites in 

 association with a supposed Permo-Carboniferous tillite in Massa- 

 chusetts. 



The generally accepted explanation is that these alternations in 

 deposition were produced by seasonal variations, the clay layer being 

 laid down in the summer, for example, and the carbonate layer in the 

 winter. 



Allen 156 has described some ribbon shales in the lower part of the 

 Chancellor formation of Upper Cambrian age in the Field Map Area. 

 The formation consists of a rather uniform calcareous and dolmitic 

 shale rich in iron. The shale weathers in many characteristic shades, 

 and while on fresh surfaces no great differences are seen, on weathered 

 surfaces the shale is characteristically banded or striped. This strip- 

 ing appears to be due to slight differences in the chemical composition 

 of the different layers, developed by weathering. The layers vary in 

 width from a fraction of an inch to one and one-half inches. Sections 

 several hundred feet thick may show this striping throughout. This 



152 Berkey, C. P., Laminated Intorglacial Clays of Grantsburg, Wisconsin, 

 Jour. Geol., vol. 13, p. 35, 1905. 



isa M. E. Wilson, Kewegama Lake Map Area, Quebec, Can. Geol. Surv. Memoir 

 39, p. 104, and plates 25 and 26, 1913. 



24th Ann. Rep. Ontario Bureau of Mines, pt. m, pp. 5-6, 1915. 



is* On Late Quaternary Time and Climate, Geol. Foren Forhandl. vol. 30, 

 p. 459, 1908. 



issSayles, R. W., Bull. Geol. Soc. Am., vol. 27, p. 110, 1916. 

 i- r >6 Allen, J. A., Geology of the Field Map Area, British Columbia and Alberta, 

 Can. Geol. Surv. Memoir 55, 1914. 



