58 NEW YORK STATE MUSEUM 



have been developed at great depth (thousands of feet) below the 

 surface. Judging by the present rate of erosion of rock masses we 

 believe that the erosion period had a duration of at least several 

 million years extending into the early Paleozoic era. 



Toward the close of this erosion period minor igneous intru- 

 sions occurred, such as are represented by the small dikes of 

 gabbro or diabase of the quadrangle. These rocks are certainly 

 much younger than the rocks already referred to as shown by their 

 occurrence in the form of dikes and by their lack of metamorphism. 

 Their fine-grained texture shows that they were cooled much nearer 

 the surface than were the syenites and granites. 



As a result of the vast erosion the whole land mass was worn 

 down to near the sea level and presented only a moderate relief. 

 Then a gradual sinking took place when the sea steadily encroached 

 upon the land and caused a deposition of one layer of sediment 

 (Paleozoic) after another upon the former land surface. The 

 whole area of the quadrangle as well as most, if not all, of the 

 Adirondack region was thus submerged. The deposition of these 

 sediments, largely derived from a wearing away of the sinking 

 land, upon the ancient gneisses has given rise to the profound un- 

 conformity now existing between the Paleozoic and Precambric. 



The first deposit to form upon the sinking Precambric rocks of 

 the quadrangle was the Potsdam (Upper Cambric) sandstone. The 

 coarse conglomerate and sandstone now seen at the base of the 

 Potsdam literally represents the boulders and sand accumulated 

 along the encroaching shore line those millions of years ago. With 

 a deepening of the water came the deposition of the alternating 

 sandstones and limestones of the Theresa formation and above 

 these the Little Falls dolomite (Upper Cambric). 



After the deposition of the Little Falls there was a gentle upward 

 oscillation of the area above sea level so that the Little Falls dolo- 

 mite suffered erosion. This old eroded surface may be seen 

 throughout the Mohawk valley and marks (by unconformity) the 

 boundary between the Cambric and Ordovicic systems. 



Next came a sinking of the land below the ocean surface when 

 the Tribes Hill (Ordovicic) limestone was formed. This was fol- 

 lowed by another gentle emergence of the land above the sea when 

 a notable amount of erosion again took place. This emergence and 

 consequent erosion is shown by the distinct unconformity now ex- 

 isting between the Tribes Hill and overlying Black River-Trenton 

 in the Mohawk valley as well as by the practical absence of the 



