and South Central Pennsylvania. 405 



Complete and well-exposed sections are not numerous 

 within any of the areas, but fortunately the development 

 of quarries along the strike of the purest beds of lime- 

 stone, which extend through the towns of Tyrone, Belle- 

 f onte and Salona, exposes the zones above and below this 

 horizon. The older quarries, where weathering has gone 

 on for some time, furnish the best general collecting 

 ground for fossils. Well exposed sections of the lower 

 Middle Ordovician are to be found in the abandoned road 

 metal quarries of the region. Unfortunately, however, 

 most of the above mentioned sections lie close to the Alle- 

 ghany Plateau under which the Ordovician limestones of 

 Pennsylvania dip. 



To the southeast of the central Pennsylvanian prov- 

 ince lies a more or less parallel band of somewhat meta- 

 morphosed Ordovician limestones and dolomites which 

 extends through Carlisle, Easton and Port Jervis. The 

 southwestern extension of the easterly belt, on the other 

 hand, the formations of which are metamorphosed, 

 passes through Chambersburg and continues as far as 

 Birmingham, Alabama. Thus the central Pennsylva- 

 nian province is the most western extension of the Ordo- 

 vician formations of the state, and it will also be seen 

 that this province occupies a middle and somewhat iso- 

 lated position within the Ordovician terranes of eastern 

 North America. 



History. 



The earliest reference to the Ordovician of central 

 Pennsylvania appears to be that of Richard C. Taylor 

 (2), who in 1835 noted the fossiliferous beds at the foot 

 of Jacks Mountain. Then there is no mention of it. until 

 the publication in 1858 of the First Geological Survey of 

 Pennsylvania by H. D. Rogers (3). In this work, on an 

 excellent colored geological map which is supposed to 

 have been compiled by the Survey but which was prob- 

 ably largely copied from an earlier map of Dr. Hender- 

 son's, the Ordovician formations are shown in light-blue 

 and green. 



The geological map. of the Second Survey (4) shows 

 little or no advance over the first, the Lower and Middle 

 Ordovician formations being still undifferentiated and 

 shown in light blue. The county reports, however, con- 

 tain many references to Middle Ordovician outcrops and 



