GEOLOGICAL IMPORTANCE OF SEDIMENTATION 553 



ILLUSTRATIVE GEOLOGICAL APPLICATIONS 

 In the preceding paper under the heading of "The Relations 

 of Continental and Marine Sedimentation through Geological 

 Time," it was concluded, not from a detailed study of the strata, 

 but entirely from the broader relations at present prevailing, that 

 at certain times in the past continental sedimentation should have 

 played an important role, especially in the form of fluviatile deposits 

 filling interior basins or displacing epicontinental seas. Having 

 made this present examination of the different methods by which 

 mud-cracks may originate, together with some of their associated 

 characteristics, it will be well to apply it as a test to the conclusions 

 of the preceding paper. If the result is a confirmation, there will 

 thus be two largely independent lines of reasoning which arrive at 

 the same result; a result in which therefore correspondingly more 

 confidence may be placed. 



MUD-CRACKED FORMATIONS OF THE PRE-CAMBRIAN 



In both northwestern Montana and northwestern Arizona occur 

 a series of predominantly arenaceous and argillaceous formations 

 of great thickness which are distinctly older than the Middle Cam- 

 brian, since these lower formations were gently folded and base- 

 leveled before the transgression of the Middle Cambrian sea. Yet 

 these terranes are remarkably free from metamorphism and still 

 retain their original characters. For this reason they are selected 

 for illustration and briefly described. It was suggested in the pre- 

 ceding article that on account of the general nature of the deposits 

 and the fact that the early Cambrian as well as the immediately 

 pre- Cambrian were periods of great continental extension, the hypoth- 

 esis of subaerial and fluviatile origin for certain formations should 

 be at least entertained until disproved. It is now proposed to 

 describe certain features of these formations in detail in order to 

 arrive at some conclusion in regard to their continental, littoral, 

 or marine origin, the conclusions being drawn after the presentation 

 of the details. 



PRE-CAMBRIAN FORMATIONS OF MONTANA 



These are described by Walcott under the title of the "Belt 

 Terrane"^ and by Willis as the "Algonkian of the Lewis and Liv- 



I Pre-Cambrian Fossiliferous Formations," Bulletin Geological Society of America, 

 Vol. X, pp. 201, 215. 



