ORIGIN OF THE SANDSTONE 247 



of composition, in the writer's opinion, not to chemical precipitation 

 of the rock, not to subsequent leaching, but rather to the unusual com- 

 pleteness of the assorting process accomplished by wind and water on a 

 sand largely derived from a previous sandstone formation. The marks 

 of water action and sedimentation are too numerous to admit the chem- 

 ical theory for an instant. As for subsequent removal by leaching, the 

 results are too capricious for so general a cause. It has not removed the 

 color stains that so commonly emphasize the structural features. It is 

 impossible that nearly all the mineral dust, even that of the quartz type, 

 should be removed by such agency and yet not attack the larger grains, 

 even enough to destroy their wind-worn finish or the stains they carry. 

 Such carbonate matter as accumulated in the sands has been removed 

 largely, it is true, but there is no evidence of any considerable quantity 

 of it ever having been present. For the removal of siliceous matter there 

 is no evidence. 



That the Saint Peter sandstone was deposited in water and preserves 

 chiefly such structures as are common to sediments is certain ; that its 

 grains fall within the range of wind transportation and show character- 

 istic wind-worn surfaces is equally clear; that the formation relationships 

 argue an extensive retreat of the sea and an erosion interval is well sup- 

 ported — these factors alone are sufficient to account for all the peculiar- 

 ities and remarkable characters of the Saint Peter, without any special 

 agency. 



Physiographic Changes 



A reasonable interpretation of the foregoing facts and observations 

 suggests the following outline of the physiographic changes of the time: 



1. Just preceding the Saint Peter epoch the sea stood far in on the 

 continental areas represented by Laurentia at the north and Appalachia 

 at the east. These were connected by an isthmus occupying the position 

 of the Frontenac axis. To the eastward lay the Atlantic ocean and the 

 gulf of Maine, in which the typical Beekmantown sediments were already 

 accumulating. To the westward, in the great Mississippian sea, dolo- 

 mites and limestones of the Magnesian series and their equivalents were 

 being deposited. 



2. With a reversal of epeirogenic movement the sea began to retreat. 

 There were some marginal disturbances, perhaps both folding and fault- 

 ing, by which arose subsequently some structural unconformity and 

 through which possibly extra supplies of sands were made available. 



3. At maximum retreat, judged by the distances to which heavy sands 

 were carried, the whole upper Mississippi valley became dry land; it 



