GEOLOGY OF SARATOGA SPRINGS AND VICINITY 59 



These are on the same line with one another and also about on line 

 with the prolongation of the West Galway fault. The throw also 

 is much the same so far as can be judged, but the direct evidence is 

 meager. 



East Galway faiilt. The evidence for the extension of this fault 

 as far as Middlegrove is fairly satisfactory. At first it shows Cana- 

 joharie shales on the downthrow side. Back from Rock City Falls 

 the Amsterdam and the Little Falls come in. On the upthrow side 

 the horizon varies but little, the surface of the slice lying very flat. 

 Beyond Middlegrove it runs into the heavy drift, but its trend would 

 be with the axis of the preglacial valley for the next few miles. 

 Farther north its occurrence is problematical, but the presence of a 

 fault is needed to explain the occurrence of the Potsdam and Theresa 

 formations at and south of Corinth, which are wholly out of adjust- 

 ment with the same formations in the eastern part of Greenfield 

 township. We think a fault must lie here, and it seems more reason- 

 able to connect it with the East Galway fault than to assume a 

 wholly separate break. The chief objection to this view is that the 

 fault south of Corinth downthrows to the west, while at East Galway 

 the downthrow is to the east. This may be explained, however, by 

 the fact that the slice of territory to the west of the fault lies very 

 flat, while to the east of the fault the rocks are more tipped, having 

 a very noticeable southwest dip. Because of this the throw steadily 

 diminishes in passing north as far as a point southeast of Porter 

 Corners, where the Theresa formation is present on both sides of 

 the fault and the throw has become zero. To the northward the 

 throw reverses and the older rocks are present on the east side of 

 the fault instead of on the west. Faults of this type, called 

 " rotatory " faults, are not very common, which is the cause for 

 greater regret, as the heavy drift-covering makes the whole matter 

 so uncertain. 



Rock City Falls fault. The small fault at Rock City Falls has 

 been described by both Darton and Prosser. It is well exposed in 

 the creek, the fall itself being practically on the fault line, 15 feet 

 of Little Falls dolomite underlying the Amsterdam on the west or 

 upthrow side, while the base of the Amsterdam is below the creek 

 level on the downthrow side. To the south of the creek also recent 

 (|uarrying of the Amsterdam has exposed the fault line excellently 

 for a short distance, though with Amsterdam limestone on both 

 sides, a little fault breccia, and with much updrag of the rock on 

 the downthrow side. The throw of the fault is only 25 to 30 feet. 



