1894-] PLANTS OF MONROE COUNTY. 35 



drift encounter first the shales of the Salina formation. These shales, 

 which contain the salt deposits and stretch from Syracuse to the 

 Niagara river, form a belt across our area quite as wide as the whole 

 Niagara formation, (Niagara, Clinton, and Medina groups). Upon 

 this meridian the total thickness of the shales, with some calcareous 

 beds, is about 600 feet. Passing southward to the lower border of 

 Monroe county the next successively higher rock, the Corniferous 

 limestone, appears. It is about 140 feet thick, of hard and enduring 

 character, and characterized by abundance of flint nodules. It has 

 resisted glaciation and weathering better than the Salina shales, and 

 so forms usually a definite escarpment at its northern edge, producing 

 cataracts or rapids in the streams. Upon this formation the drift is 

 thin, and in some localities, particularly near Caledonia and LeRoy, in 

 Genesee county, there are extensive superficial quarries. The lime- 

 stone outcrops over a belt of territory two to five miles wide. 



The southern quarter of the area has the shales and sandstones 

 of the Hamilton group as the superficial rock. These shales are dark 

 and carbonaceous, and frequently yield rock-gas. With a capping of 

 Portage sandstone they form the high table-land of the lake region 

 south of the mapped area. 



Pleistocene Drift. — The superficial geology of the region has not 

 been described in detail, and will be treated here only in a general 

 way. During the millions of years following the deposition of the 

 Devonian rocks the region was continuously exposed to destructive 

 atmospheric agencies, and by atmospheric decay and stream erosion 

 a great thickne s of rocks had doubtless been removed from this 

 area.* At the close of the Pliocene period the climate, which had 

 been slowly growing colder, produced a great accumulation of snow 

 and ice over Canada and the north-eastern United States, and the 

 subaerial denudation was changed to subglacial. The superficial 

 decomposed rocks were crushed and removed by the southward 

 moving ice-sheet, the old drainage channels were largely filled with 

 the debris, and the final removal of the ice left a sheet of glacial drift 

 over the whole territory. 



The great glacier not only eroded the decomposed and exposed' 

 rocks of the region and spread the wreckage over the area to the 

 south, but it brought in from the north, or north-east, the complex 

 materials of the crystalline rocks of the Archean areas. Thus the 



♦Proc. Roch. Acad. Science, Vol. II, p. 221. 

 5, Proc. Roch. Acad, of Sc, Vol. 3, January, 1896. 



