FIFTH REPORT OF THE DIRECTOR I908 



167 



Adirondack Precambrics under the Potsdam formation, by rocks 

 of the same kind. 



Some most interesting examples of the style of inliers last men- 

 tioned are observed in the Little Falls sheet, mapped by Professor 

 Cushing. We here especially note 

 the Middleville inlier [see text 

 .fig. 3, 4]. This is an outcrop of 

 syenite rocks in the bottom of the 

 deep valley of West Canada creek. 

 Although it is obvious that the cor- 

 rasion by the river is the final 

 cause of the exposure of this rock 

 mass, it follows also from the ab- 

 sence of exposures of the same 

 rock farther up and down the 

 valley, that the ' syenite here pro- 

 trudes into the overlying and 

 surrounding Beekmantown beds in 

 consequence of the irregularity of FlG - 3 

 the surface of the Adirondack 



OOO 

 OOO 



in Beekmantown limestone j 

 Middleville, N.Y. Scale i m. 



plateau over which the Beekman- = 1 in - 

 town sea advanced. The syenite is of the same age as the other 

 syenite intrusions associated with the Grenville series of the Adiron- 

 dacks and a portion of the Adirondack massive. Underground it is 



Fig. 



Section of inlier at Middleville, N. Y. 



Beekmantown 



/\ y\ y\ 







\ /\ /\ / 



Precambric 











connected with the larger and better known Little Falls inlier con- 

 sisting of a like syenite. Since the latter, however, is much involved 

 in strong faults and obviously owes much of its prominence to 

 " horst " structure, we will mention it more fully under that 

 caption. 



A much smaller group of inliers of Precambric rocks in the Pots- 

 dam sandstone similar to those on the northwest and north side of 

 the Adirondack's, is also found on the east side, about Port Henry. 

 The Potsdam there rests as a thin veneer on a steeply tilted fault 

 block and in several places knolls of Precambric rocks which 

 clearly were once small islands in the shallow Potsdam sea, project 

 above the much eroded Potsdam beds [see text fig. 5]. 



