along Half Moon Run. These sections extend across the 

 Valley of Lower Silurian Limestone, with beds of brown 

 hematite iron-ore, and across the bounding mountains of the 

 Middle Silurian Sandstone, Bald Eagle Mountain on the 

 west, and Tussey Mountain on the east, the great anticlinal 

 upthrow of Bellefont being seen in all three sections at the 

 east foot of Bald Eagle Mountain, the Limestones dipping 

 east away from the fault at a uniform dip of about 54°. 



He then explained the theoretical difficulties which have 

 hitherto beset the dynamic questions raised by a phenomenon 

 of this kind, an overthrown and faulted anticlinal ; especi- 

 ally the question why a dip of just above 64° should follow 

 one side of the fault for many miles, when the rocks on the 

 other side of the fault stood vertical. 



This question he thought he had just succeeded in settling 

 by a discovery which resulted from the construction of a 

 fourth section, which he exhibited, extending from the Bald 

 Eagle Mountain westward to the summit of the Alleghany 

 Mountain, taking in the vertical Middle Silurian rocks, the 

 steeply inclined Upper Silurians, the Devonians dipping 

 regularly less and less (from 28° to 8° where observed at 

 different points along the section), and the almost horizontal 

 Lower Coal Measures at the summit of the Alleghany 

 Mountain. 



By a system of co-ordinates, the exact curve of the up- 

 throw on the western side of the Great Fault w^as displayed, 

 using the observed dips along the line of section as elements 

 of construction. The country west of the dip of 15° was as- 

 sumed to be in its original condition. East of this point, or 

 of the " hypothetical limit of stability," the steeply upturned 

 strata were supposed to slide upon each other with a shear- 

 ing motion. The basset edges of the vertical strata must be 

 considered as rising in steps above each other westward at 

 the plane of fault, the slope thus obtained facing the east, 

 many thousand feet in the air, over the Bald Eagle Mountain. 



On calculating the angle of this slope, which is not a per- 

 fect plane, but a slightly curved surface, it turned out to be 



