656 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



At least one and probably two anticlinal axes that at various times 

 interfered with and perhaps prohibited the free westward expansion of 

 the Appalachian sea doubtless extended southward from the Adirondack 

 land mass into Pennsylvania during the greater part of Canadian time. 

 To the more easterly one of these may be ascribed the marked differences 

 in lithologic characters of the middle and upper divisions of this great 

 formation observed in comparing the section at Chambersburg with 

 those to the west in the vicinity of Mercersburg. The second axis sepa- 

 rated the Mercersburg trough from the Bellefonte basin. The broad 

 Harrisburg axis (see page 563), which trends northwest and southeast, 

 exerted an even more important effect on Canadian seas in that it pro- 

 hibited late Beekmantown deposition in these east Pennsylvania valleys 

 to the north of Harrisburg. Only the early Canadian Tribes Hill and 

 the Stonehenge member or formation extends across all these barriers, 

 this zone being as clearly recognizable in the Mohawk Valley of New 

 York as in the eastern and central Pennsylvanian sections. The middle 

 and upper divisions, however, vary conspicuously in lithic characters 

 from east to west. The same barriers became even more efficient in mid- 

 dle Ordovician time. 



Canadian deposits in central Pennsylvania. — The Canadian is devel- 

 oped to extraordinary thickness in Nittany and other valleys in central 

 Pennsylvania. According to present information, the maximum thick- 

 ness for the region, about 4,230 feet, occurs at Bellefonte. The section 

 here is divisible into four formations, the two lower of which pinch out 

 by overlap southward. Several descriptions of the section at Bellefonte, 

 which is almost continuously exposed, have been published, the last and 

 by far the best by Collie.^^ 



I have twice visited this locality, verified Collie's measurements, and 

 collected some of the fossils from horizons pointed out by him. Besides 

 discovering several additional fossiliferous zones, the section has been 

 revised and the Ozarkian beds, which rise to the surface in the axis of 

 the great anticline, have been satisfactorily separated from the lowest of 

 the Canadian formations. As the section at Bellefonte affords the maxi- 

 mum development of the Canadian system in the Appalachian region 

 and contains a greater number of abundantly fossiliferous zones, it seems 

 worth while to describe it in moderate detail. This is all the more 

 desirable because the formations into which it is readily divisible have 

 been incorporated in the time scale. 



"^G. L. Collie: Bull. Geol. Soc. America, vol. 14, 190.3, pp. 410-411. 



