146 ROCHESTER ACADEMY OF SCIENCE. 



to the material derived thus close at hand, while at the east the 

 shallow platform is really a wave-cut terrace, not a refilling. 



Evolution of the Valley. 



In the endeavor to reincarnate the sequence of events in the 

 lower Irondequoit valley, these fall readily into three principal 

 stages, which, without forcing the simile, may be conveniently desig- 

 nated as follows : — 



I. Pre-natal period, during which the conditions were being 

 made ready for the birth of the valley ; 



II. Life-span of the valley as the product of a great river, 

 brought to an untimely close by the glacial occupation ; 



III. Post-mortem period, in which the valley is not merely 

 dead but buried, as we have been considering, under a pall of 

 Pleistocene weaving in which the modern Irondequoit river has 

 made a big rent. 



Perhaps to these we should append its possible resurrection in 

 time to come. 



Early history. The story of the "pre-natal" period is the story 

 of Paleozoic sedimentation, whose details are not germane to this 

 paper, and of the slow uplift of those sediments into a coastal plain 

 whose inner margin lapped along the Canadian- Adirondack old- 

 land on the north and northeast while its outer edge disappeared 

 southward and southwestward under the slowly retiring Mississip- 

 pian Sea. Across this foreland the rivers of Canada, draining the 

 old-land, prolonged themselves in roughly parallel courses to the 

 sea. Thus was begun the process of valley-making on the surface 

 of this plain. ^' 



The story of the "life-span" of the valley thus born is the story 

 of the development of that great trench in the underlying strata 

 which is the subject of our forthcoming paper on the rock topog- 

 raphy, wherein its character and proportions are visualized. Com- 

 plete unanimity as to the significance and history of this rock trench 

 does not obtain, but the following sketch may be considered as har- 

 monizing most of the interpretations. The original rivers of this 



13. See Dr. A. W. Gral)au: Bulletin 4.5 N. Y. S. Museum, pp. 37-47. 



