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WILLIAM J. MILLER 



of Grenville strata is certainly not due to severe lateral compression, 

 nor is there, in any part of the quadrangle, evidence of highly folded 

 or compressed Grenville strata. 



In the northwestern part of the Thirteenth Lake quadrangle 

 the writer has examined Chimney Mountain, which is a mass of 

 granitic syenite rising fully 900 feet above a valley on the west. 

 Perfectly bedded Grenville rocks with dip of 50 lap over the whole 

 western face of the mountain of igneous rock, and it seems certain 

 that the tilt of the strata was produced by the rise of the magma. 



We are thus led to conclude that none of the published Adiron- 

 dack geologic maps or available data afford any reason to believe 

 that the Grenville strata were ever profoundly folded or compressed. 

 There is, however, much tilting on large and small scales and some 

 very moderate folding. Such structures may be readily accounted 

 for simply by the irregular intrusion or upwelling of great bodies 

 of more or less plastic magma which broke up, tilted, and lifted or 

 domed the masses of Grenville. 



Grenville structure in the Thousand Islands and Ontario regions. — 

 The Thousand Islands district forms the connecting link between 

 the Adirondack and Canadian pre-Cambrian areas, and lies to one 

 side of the region discussed in this paper. Having recently studied 

 the Thousand Islands district, Cushing says: "The Grenville 

 beds are now found for most part in highly inclined condition, dips 

 of less than 45 being relatively rare, while those approaching 



verticality are common It has also been shown that 



the dip is not everywhere in the same direction, but that, with the 

 general direction of strike to the northeast-southwest, the dip, 

 while prevalently to the northwest, becomes at times southeast. 

 .... The highly tilted condition of the rock series, and the 

 changing dips seem certainly indicative of folding." 1 He then 

 describes a prominent belt of Grenville strata which he believes 

 has a synclinal structure. But, accepting the existence of this 

 syncline. does such a structure prove. the region to have been sub- 

 jected to an intense force of compression ? Large bodies of granite 

 bound this Grenville belt on either side, and it is quite conceivable 

 that the uplifting effect of the intruding masses, possibly accom- 



1 H. P. Cushing, New York State Mus. Bull., No. 145, 1910, p. 109. 



