GEOLOGICAL IMPORTANCE OF SEDIMENTATION 319 



the latter to widely transgress the base-leveled land. Therefore, in 

 the topographic old age of a continent shallow-water marine deposits 

 should attain a maximum. 



This cycle of relations between continental and marine sedimenta- 

 tion is then applied to geological history, especially to the late Pro- 

 terozoic and Paleozoic, and it is concluded from the general geo- 

 graphic relations of certain epochs, as understood at the present time, 

 that notable subaerial deposits of river waste might be expected to 

 occur within those particular epochs. 



The preceding discussion prepares the ground for a second por- 

 tion, concerned with the detailed consideration of the significance of 

 mud-cracks in association with other features, as indicating the con- 

 tinental and alluvial nature of certain deposits. Since mud-cracks 

 also occur in littoral deposits, the necessity for the preceding quanti- 

 tative study is seen. It is concluded that, next to coal-beds formed 

 in situ or an abundance of land fossils belonging to the animal king- 

 dom ; in association with certain other easily recognized features mud- 

 cracks form one of the surest indications of the continental origin of 

 argillaceous deposits. The structure is also seen to most commonly 

 originate under climatic conditions where the other tests are apt to 

 fail. It is not contended that mud-cracked littoral deposits may not 

 also form, but it would appear that they should be relatively rare. 



This conclusion stands in opposition to the interpretation given 

 to mud-cracks in the standard textbooks, where they are ascribed to 

 the drying of tidal flats and regarded as evidences of shallow seas; 

 the flood-plain origin, if noticed at all, being given second place. 



On account of this divergence from the prevailing interpretation, 

 attention is called to the desirability of confirming or modifying the 

 present conclusion. 



Finally, the criteria developed as to the significance of mud- cracks 

 are applied in detail to the Belt terrane of Montana and the Unkar 

 and Chuar terranes of the Grand Canyon, both of late Proterozoic 

 age. This furnishes an example of the use of the criteria, and at the 

 same time draws conclusions in regard to the origin of these forma- 

 tions, which in general are barren of both animal and plant fossils, 

 and therefore lack the usual guides for arriving at their origin. 



It is concluded that the Belt gives an illustration of two sedimen- 



