726 PRINCIPLES OF STRATIGRAPHY 



thorough sorting by the encroaching waters, a nearly pure silicar- 

 enyte may come to rest directly upon the eroded surface of the 

 crystalline old land. This is finely shown in the basal Palaeozoic 

 contact in portions of the Front Range of the Rocky Mountains, 

 where a nearly pure cjuartz sandstone rests on an almost perfectly 

 even erosion surface of granite. (Crosby-2.) Where atmospheric 

 agencies have been sufficiently active to disintegrate a granite 

 surface, without, however, reducing the feldspar to clay, the basal 

 sandstone will be a feldspathic arenyte or arkose. Again examples 

 are known where decomposition has affected the underlying crystal- 

 line old land to a considerable extent, but where little sorting of 

 material was accomplished by the transgressing sea,' so that the 

 basal bed is a highly argillaceous arenyte. The contact of this with 

 tlie underlying crystalline basement rock will in consequence not 

 be a sharp one, the crystalline rock grading through a decomposed 

 zone into the overlying sandstone. An example of this indefinite 

 type of contact is seen on Presque Isle near Marquette, Michigan, 

 where the Lake Superior sandstone passes downward into a rock 

 produced by the consolidation of the undisturbed disintegrated sur- 

 face of the basal peridotite. 



A consideration of the progressive landward migration of the 

 coarser deposits under uniform conditions will lead to the recogni- 

 tion that the changes in any given bed, from the shore seaward, 

 will be exactly duplicated by the changes in a given vertical sec- 

 tion from the base upward.* For it will be seen that the coarse 

 bed deposited directly upon the old sea-floor of crystalline rock is 

 succeeded upward by a somewhat finer bed, since the zone of dep- 

 osition of the coarse material has, by the continuous subsidence, 

 migrated further landward. Thus, as shown in the annexed figure 

 (Fig. 144), the lowest coarse deposits of bed (a)' which form the 

 shore zone of that bed are succeeded vertically by the finer deposits 

 of bed (b) made at a somewhat greater distance from the new 

 shore. At this new shore (of bed b) coarse deposits are accumu- 

 lating, but they are beyond the belt of the former deposition of 

 coarse material. Again an advance of the shore to c transfers 

 the shore belt of coarse (rudaceous) deposition in the same direc- 

 tion and by the same amount. Consequently the belt of arenaceous 

 deposits of bed c is likewise transferred shoreward and comes to 



* The variation in texture of deposits due to storms and the corresponding 

 change in the power of waves and currents discussed in a preceding chapter, 

 are here left out of consideration, since they will at best produce only minor 

 variations in the strata. The present discussion deals with formations on a large 

 scale. 



