558 STUDIES FOR STUDENTS 



the lime and magnesia of a primitive rock mass would contain but 

 about one-tenth of the original, and in volume, allowing for the car- 

 bon dioxide in combination with bases, would roughly represent 

 about a fifth of the original. In this case, however, since the material 

 has been deposited from solution, it does not signify a necessary 

 origin from contiguous land masses. 



The average igneous rock, containing, according to Clarke, lo 

 per cent, of quartz, a pure quartzite will represent less than one- 

 eighth of the original rock mass, but an argillite, containing variable 

 amounts of the original quartz and additional water and carbon 

 dioxide combined with the bases, represents a far higher, but indefi- 

 nite, proportion of the original rock mass. Quartzites and argil- 

 lites, however, since they cannot be transported across deep bodies 

 of water imply contiguous land. The great thickness and simi- 

 larity of the arenaceous and argillaceous formations over a wide 

 area point to an originally still more widely spread character, since 

 there is no indication that these districts were near the original 

 limits. But their volume indicates deep erosion of a correspond- 

 ingly extensive contiguous land. The formations do not show the 

 local variations and conglomeratic nature which would indicate the 

 erosion of a nearby mountain range, and therefore the denudation 

 must have taken place from a wide area. The similar formations 

 which are known to exist in British Columbia, Utah, Nevada, Cali- 

 fornia, and Arizona emphasize still further the profound erosion 

 of widespread adjacent land masses of late pre-Cambrian time. 



Thus a detailed examination of the composition and texture of 

 these Montana formations allows inferences confirming for this 

 region the statements made in the previous article from more general 

 grounds concerning the wide development of the continents in the 

 later pre-Cambrian times. 



In regard to the topographic and sedimentary cycles expressed 

 by the succession and character of the formations it is seen that the 

 two great limestones represent long- enduring incursions of the sea, 

 while the quartzites and argillites represent the uplift and erosion of 

 neighboring lands of large area. 



The Neihart quartzite.— The cleanness and partially deferrized 

 character of the basal formation, the Neihart quartzite, indicates 

 shallow water off-shore deposit, subject to the prolonged sorting 



