GEOLOGICAL STRUCTURE OF THE COUNTY. b . od 



northern trough in the next 30 miles ; or exactly three 

 times as fast. 



The cause of this difference is seen in the fact that the 

 tico sides of the northern trough are symmetrical ; whereas 

 the south stele of the southern trough is turned up and 

 pressed over inwards ; narrowing, sharpening and shorten- 

 ing the trough all at the same time and by the same opera- 

 tion. See page plate III, page 30, Figs. 1, 2, 3, 4. 



Sub ■plications of the troughs and arches. 



The structure however, is by no means so simple as the 

 foregoing general idealized sketch would lead the reader to 

 suppose, except along the Susquehanna river, the eastern 

 border of the county, and for a few miles inland to the 

 west. After that it becomes complicated by many excep- 

 tional details, which are, each of them, of great local im- 

 portance, and which it will be the business of this report to 

 describe. Meanwhile, a general idea of these complications 

 must here be given. 



Along the Susquehanna river there are but (wo great 

 troughs, each simple in its structure, and one great arch 

 between them (represented by Half Falls mountain, 1 mile 

 south of Montgomery,) itself also quite simple in its struc- 

 ture. 



On the Juniata river the northern trough still retains its 

 simplicity, but the middle arch (or grand anticlinal) at 

 Baileysburg begins to exhibit irregularities, being both 

 double and faulted. 



Nine miles west of the Juniata, on a line through Bloom- 

 field, the middle arch is not only complicated three times 

 on each side of its included little basin, but also faulted. 

 See page plate IV, page 32, Fig. 1. 



Three miles west of Bloom field, the little basin has be- 

 come enlarged by the absorption of some of the plications 

 on the north side of it, and by the introduction of new rolls 

 See Fig. 2. 



Five mil^s further west, a cross-section from Green Park 

 to Bridgeport and Oakgrove shows the little basin compli- 

 cated four times, as in Fig. 3. 

 3 F 2 . 



