54 NEW YORK STATE MUSEUM 



Still more recent work on some of the faults which reach the 

 Saratoga quadrangle has been done by Prosser, Cumings and Fisher 

 in their mapping of the Hoffmans Ferry fault across the Amster- 

 dam quadrangle, near the north edge of which it gives rise to two 

 branches ; and by Miller in his mapping of these and other faults ; 

 of the Broadalbin quadrangle/ Of these only the Hoffmans Ferry j 

 fault and branches pass over on the Saratoga quadrangle, while ! 

 other and more easterly faults come in. - 1 



All the larger of these Mohawk faults have a trend somewhat > 

 to the east of north, and a rude parallelism with one another. To 

 the south they all run into the great thickness of upper Ordovicic ! 

 shales, and as soon as these come to form the surface rocks on both 

 sides of the fault, it is exceedingly difficult to trace the dislocation ! 

 farther. We do not as yet know whether they die out in the shales ; 

 or not. To the north the faults run into the Precambric rocks; 

 and so soon as they have these on both sides of the fault, a similar ' 

 difficulty arises in the effort to trace them farther. But they seem 

 to run entirely across the Adirondack region from south to north 

 and diminish in frequency toward the west. Hence it results that 

 they abound in the southeast border of the region and are practi- 

 cally absent on the northwest. With their parallelism they divide 

 the region into a great series of rock slices or segments which have 

 shifted up and down past one another and broken the continuity 

 of the rock formations. 



The fault planes are seldom visible but such evidence as we have 

 indicates that they are nearly vertical breaks and are all of the type 

 known as normal faults. In the great majority of them the east side 

 has dropped in level relatively to the west side, but in a few of 

 them the reverse is true. ^ 



In addition to the relative displacements of the adjacent slices 

 along the fault planes, the upper surfaces of the slices have usually 

 received a tilt toward the west, each slice thus constituting the up- 

 throw side of a fault along its east edge, and the downthrow side 

 of the next fault along its west edge, as illustrated in figure 4. 



The faults. Two of the great faults of the Mohawk region, the 

 Hoffmans Ferry (much better abbreviated to Hoffmans) and the 

 McGregor fault, the latter here named for the first time, cross the 

 Saratoga quadrangle. The remaining faults mapped are branches 

 of these two great breaks. 



1 N. Y. State Mus. Bui. 34 and map; Bui. 153 and map. 



