560 STUDIES FOR STUDENTS 



marked sameness in color and a similar repetition of argillaceous 

 and arenaceous beds. The areal extent, the fineness and evenness 

 of grain, combined with the evidences of aridity, bespeak an exten- 

 sive delta fan, comparable in size and climatic environment to those 

 of certain of the larger Asiatic rivers of the present time. 



The lack of knowledge as to the extent and relations of the forma- 

 tion will not allow of a closer comparison, but it is seen that it repre- 

 sents the continental culmination of the sedimentary cycle as the 

 preceding Newland and Altyn limestones represented the opposite 

 or marine phase. 



The cycle appears to.be less dependent here upon a mere trans- 

 gression and recession of the sea as the active agent, than upon the 

 wasting-away and the rejuvenation of adjacent land masses, which, 

 upon being re- elevated, supply such a flood of sediment as to crowd 

 back and dispossess the sea, the subsidence of the geosyncline going 

 forward more or less continuously but at a variable rate. 



The succeeding formations of this terrane indicate a repetition 

 of this cycle, the Empire shales, showing greens and grays and 

 passing into the upper limestones, doubtless represent the submarine 

 deposits made during the transgression of the sea across the subsid- 

 ing former delta surface. Then followed several thousand feet of 

 limestone formation. This is largely thin bedded and shaly in the 

 upper portions in the Helena region and various features described 

 elsewhere suggest that it was largely accumulated in a shallow sea. 

 This limestone was succeeded in places by quartzites which suggest 

 wave- sorted deposits, and finally by a deep red mud-cracked series 

 of argillites, somewhat similar to the Spokane- Grinnell formation, 

 suggestive once more of land deposition under conditions of aridity. 

 The thickness of this formation varies widely, the upper surface 

 being a base-level erosion surface upon which is superimposed the 

 Middle Cambrian marine transgression across a far-reaching and 

 topographically ancient land. 



THE GRAND CANYON SERIES OF ARIZONA 



Leaving the preceding region and passing to the Grand Canyon 

 of Arizona, some 750 miles south of Helena and some 950 miles 

 south of the international boundary, a different series of rocks is 



