EVALUATION OF STnATTOT^APllK' CRTTERTA 42f> 



suggestive of terrestrial deposition, and indicate, furthermore, a large 

 development of swamp and pond conditions under a normally humid 

 climatic condition. 



Begular handing in niudstones — A climatic record in bottomset 

 beds. — The fourth case of variegated beds consists of uniform fine- 

 grained shales in which the strata show marked variations in color. 

 Bedding may be practically absent save as marked by color. The varia- 

 tions are due either to changes in the carbon content, low carbon being 

 apt to be associated with more siliceous bands, or changes in the state of 

 the iron oxide, green and red bands making up the rock. Such varia- 

 tions are best observed in slates which cleave across the bedding and 

 thereby show the latter as bands of darker and lighter colors. The ma- 

 terial was originally clay, which settled slowly from suspension on a 

 bottom presumably not affected by waves. They are typically marine 

 deposits, and the rhythmic character of the alternation may be due to 

 changes in currents or in the shifting of river mouths, but the regularity 

 of the recurrence in many instances and its dependence on a varying 

 state of oxidation is suggestive of a climatic rather than a geographic 

 cause. In bottomset beds such a climatic rhythm may be expected to 

 record itself by a variation in color. In open seas the alternation is 

 marked more commonly by a change in the proportions of shale and 

 limestone. The rhythm is usually from a few inches to a few feet in 

 thickness, and such oscillations are of rather characteribtic occurrence in 

 shaly limestones through geological time. They are to be sharply sepa- 

 rated in significance from those variegations related to the physiographic 

 controls of shifting channels and swamps in delta and floodplain depo- 

 sition. If a climatic origin for such regular banding shall become defi- 

 nitely established the phenomenon is of high interest, for in that case 

 there is seen to exist a detailed though fragmentary record of short 

 period climatic fluctuations running back through the geologic ages and 

 indicating that oscillations about the average for the place and time are 

 and always have been characteristic features of terrestrial climates. Such 

 rhythmic changes of shorter and longer periods are indeed suggested by 

 the behavior of modern glaciers and the retreatal moraines left by the 

 extinct ice-sheets of the Pleistocene. 



A striking example, of such red and green rhythmic color banding in 

 slates has been recently studied by Dr. D. D. Cairnes, of the Canadian 

 Geological Survey, who has kindly furnished the following description 

 of them for incorporation into this paper : 



