566 STUDIES FOR STUDENTS 



origin. In regard to the upper lavas of the Unkar terrane Walcott 

 states : 



The first coulee flowed over the level ocean bed, in which 5,000 feet of sedi- 

 ment, that now forms a reddish-brown sandstone, had accumulated on the up- 

 turned and eroded edges of the Archean, the few layers of limestone and the one 

 flow of lava, 150 feet in thickness near the base scarcely serving to break the 

 great sandstone series.' 



Again the author states that 



The wide distribution of thin layers of sandstone, shale, etc., of uniform thickness 

 over considerable areas indicates a relatively smooth sea bed at the time of the 

 spreading of the first sheet of (the upper) lava over it; and that the sea was 

 shallow, is shown by ripple-marks and the filling of sun -cracks.^ 



CONCLUSION ON THE NATURE' OF THE PRE-CAMBRIAN SEDI- 

 MENTATION 



In the absence of personal observation with the particular problem 

 in mind any other interpretation than that given by Walcott should 

 be held with reservation, but it has been shown that in view of the 

 highly oxidized character of the sandstones of the Unkar terrane, 

 and mud-cracks frequently found in the shaly beds, that the presump- 

 tion is in favor of a continental origin and the burden of proof is 

 rather upon those who would give the marine interpretation. 



The discussion of these pre- Cambrian deposits but especially 

 of the Montana occurrences, shows how completely in accord is the 

 hypothesis of the dominant flood-plain origin of mud-cracks with 

 the other marks of subaerial deposition in an arid climate. The 

 mud-cracks are confined to just such formations as from other char- 

 acteristics suggest a flood- plain origin and these formations are usually 

 separated from the deposits of limestone by transitional formations 

 which differ in color, in character, and in the absence of mud-cracks, 

 suggesting the true submarine deposits originating between the shore 

 and the open sea. 



Assuming that a strong case has been made out for the continental 

 origin of certain of these pre-Cambrian formations, it is seen that 

 in the two regions examined the conclusion is justified which was 

 reached from general considerations in the preceding paper — that 

 the late pre-Cambrian being an aeon of wide continental extension 

 should show in its epicontinental deposits a considerable proportion 



I P. 504. 2 P. 517. 



