438 



PKOCEEDINGS QF OTTAWA MEETING. 



in the displacement, and I could notworkoul their relations. The relations below 

 the cement wedge are not fully exposed, but there are scattered outcrops exhibit- 

 ing the beds and their dips. 



The overthrust west of South Bethlehem is in the gentle flexures near the region 

 in which the Helderberg formations pitch up to the northward and extend to the 

 northwestward out of the flexed belt. 



The characteristics of this overthrust are the fault in the hard, massive beds of 

 pentamerus limestone and an " underturned " flexure in the thin-bedded lower 

 limestone, involving the subjacent soft Hudson slates. 



The overthrust is exposed only on Sprayt creek, which it crosses at an old mill 

 about three-quarters of a mile west-southwest of the village. Its trend is approxi- 

 mately north and south, but it does not appear to extend for any great distance. 

 The relations al the mill are illustrated in the accompanying figures, in which the 

 features above the broken-line portions of the sections are exposed in the bed and 

 banks of the creek. It is unfortunate that the exposures are not more complete, 

 but sufficient is in sight, I believe, to substantiate the interpretation I have given 

 in the figures. Only the upper surface of the crumple is exposed in the limestone, 

 but the greater part of the fault-plane is visible in the south bank of the creek 



Figure 2. — Cross-section of Overthrust West of South Bethlehem, Albany County, New Fork. 

 Exposure on south bank of Sprayt creek, looking north (reversed). 



above the dam. The overturned synclinal of slates and lowest limestone bed is 

 clearly exposed in the base of the high bank on the south side of the creek, where 

 the slate is seen to be excessively crumpled and its original bedding and cleavage 

 planes obliterated. In the north bank, under the mill, the exposure is less ex- 

 tensive, but, as is shown in figure 3, essentially similar relations exist. 



The mechanism of this overthrust is, 1 believe, not di Hi cult to understand. 1 

 have represented the hypothesis of its development in the diagrams infigure'3: 

 I, the first stage ; II, the second, and the present conditions the third. The broken 

 line on I indicates the line of weakness, the arrow the direction of thrust. The 

 fault sheared diagonally through the hard, massive beds of pentamerus limestone, 

 hut the softer, thin-bedded, underlying limestones in moving forward with the 

 thrust were not fractured, but folded downward and backward into the soft shales 



