84 NEW YORK STATE MUSEUM 



mudrock is the result of the grinding up of the black Georgian 

 shale and of Snake Hill and Normanskill *shales on both sides of 

 the thrust fault. 



Tornebohm (1896) has first shown how the rocks are ground 

 into flour along the great overthrust planes. He terms this flour 

 " Friktionsbrei " (mylonite) stating that it served as " Schmier- 

 mittel " (lubricating substance) during the overthrusting and that 

 its thickness depends on the obstacles in the mass that is overridden.dH 

 In the French central plateau these masses are said to reach several ' 

 hundred meters in thickness. 



That in Bald mountain the conditions that rest in the resistance 

 of the underground were especially favorable for the accumula- 

 tion of the mylonite, is distinctly shown by the bulging up of the 

 Bald mountain limestone mass there, the limestone, together with 

 the overlying Georgian rocks, descending north and south of the 

 face of the Bald mountain quarry, away from the mountain. 



THE NORMANSKILL SHALE 



This graptolite shale which has received its name from the ex- 

 posure at the Normans kill at Albany, forms two belts on the 

 Schuylerville quadrangle, one, entirely surrounded by Snake Hill 

 beds, coming up from the Cohoes quadrangle and terminating near 

 the mouth of the Batten kill, and another south of the Georgian 

 overthrust mass, culminating in Willard mountain. 



As in the entire shale belt in the Hudson River valley, the 

 greater part of the Normanskill formation consists of blue to' 

 gray, mostly argillaceous, often more or less sandy shales, with 

 thin intercalations of black, highly carbonaceous graptolitiferous 

 and frequently pyritif erous shales ; the lighter bluish gray and black 

 shales often giving the rock a banded appearance in the common 

 edgewise view. 



Where these shales are brought up from such depths that they 

 are still fresh and unaffected by surface weathering and frost, 

 they appear quite different, as more or less compact bluish gray 

 and black mudrocks. Considerable material of this character was 

 seen at the new canal locks above Schuylerville and in other places. 



The shales of the Normanskill formation include, however, two 

 other kinds of rock in such quantities that their frequent appear- 

 ance in outcrops can be considered as quite characteristic of the 

 formation. These are the '' white weathering cherty beds " and 

 the grit. 



