46 NEW YORK STATE MUSEUM 



river and also at several places along the brook in the valley just 

 mentioned. Nothing is known regarding the upthrow side or 

 amount of displacement. It is possible that this fault may extend 

 farther westward through the valley south of Blue ridge. 



It is quite possible that other minor zones of fracture occur 

 within the quadrangle but, if so, they are either effectually con- 

 cealed under heavy drift or escaped detection in the rough, densely 

 wooded country. 



Irregular Surface of the Syenite-Granite Intrusive Body 

 Since the Blue Mountain quadrangle lies in the rugged moun- 

 tainous district west of that portion of the Adirondacks which is 

 profoundly affected by comparatively recent faulting and contains 

 considerable areas of Grenville strata through which the great in- 

 trusions of syenite-granite have taken place, an excellent oppor- 

 tunity is afforded to study the character of the surfaces of the 

 great bathylithic masses. 



A glance at the southern half of the accompanying geologic map 

 will reveal the fact that mountains of syenite or granite frequently 

 rise conspicuously above the masses of Grenville. Differences in 

 altitude, often quite abrupt, between syenite or granite and Gren- 

 ville commonly range from a few hundred to 2000 or more feet. 

 Such marked differences in altitude must be accounted for in 

 either of two ways: (i) by faulting, whereby the Grenville has 

 been relatively dropped down with respect to the igneous masses ; 

 or (2) by irregularities on the surfaces of the igneous m.asses pro- 

 duced during the process of intrusion. That faulting can not be 

 invoked as an explanation in many cases at least is perfectly clear 

 by the fact that direct evidence for faulting is absent from many 

 places where outcrops are good at critical localities, and also by the 

 fact that certain mountain masses of syenite or granite are so 

 nearly surrounded by Grenville strata as to preclude the explanation 

 by faulting. A few concrete examples will serve to prove that the 

 bathylithic surfaces have very notable original irregularities. 



Just east and northeast of Rock lake a mountain of granite rises 

 fully 600 feet above the Grenville which latter reaches continuously 

 almost two-thirds of the w^ay around the mountain. Even if we 

 grant the possibility of a fault on one side (though there is not the 

 slightest field evidence for it), we are still forced to conclude that 

 the magma rose at least a few hundred feet above the Grenville now 



