GEOLOGY OP THE VICINITY OF LITTLE FALLS 39 



cellent work. There is therefore much in the way of details to be 

 added to his description. 



South of the Mohawk the topography gives no aid in the loca- 

 tion of the fault line, as is the case at, and north of, the river. 

 It has been traced for only some 3 miles in this direction, though 

 it ought to be traceable beyond the map limits provided rock out- 

 crops are forthcoming in that direction. Where last seen, its 

 throw is sufficiently large to guarantee that it must extend some 

 distance farther south. 



North of the river the fault can be followed with great accuracy 

 for 3 miles, both as a topographic feature and because of abund- 

 ant rock outcrops. The actual fault plane is indeed exposed at 

 several localities. Beyond, the topography locates it for 4 miles 

 more, though there is a great scarcity of rock exposures on the 

 downthrow side. Still beyond, up to the point where it passes be- 

 yond the limits of the map, its position can not be accurately 

 located, since outcrops wholly fail on the downthrow side, the 

 drift covering is very heavy on both sides, and the topography 

 gives little assistance. Darton has mapped it for some distance 

 farther, but the writer has not been over the ground. 



The fault plane approaches verticality, and the downthrow is to 

 the east [see accompanying maps and sections]. Darton estimated 

 the throw of the fault at 310 feet, which is accurate for the spot 

 where the measurement was made, but the place proves to have 

 been unfortunately chosen, as will shortly appear. Prosser's ac- 

 curately measured section at Little Falls furnishes the necessary 

 data for estimating the throw there. The pre-Cambrian rocks 

 are at the surface west of, and the Utica shales east of the fault 

 line. The entire Beekmantown and Trenton, approximating 550 

 feet in thickness, are thrown out. In addition, the pre-Cambrian 

 rocks rise to 200 feet above the river level at the fault line, while 

 the Utica shale is at the river level on the east side, so that this 

 200 feet must be added to the other, giving 750 feet. In addition 

 again an unknown thickness of Utica shale must be added. The 

 Utica is heavily dragged upward near the fault, and, to obtain the 

 actual throw, it should be flattened out. Two miles east of the 



