38 CD. Walcott — Cambrian Rocks of Pennsylvania. 



brian age. We next proceeded to the eastern side of the 

 county, where the southern division of the limestone passes 

 into Chester county. An examination was made of the lower 

 quartzites and the superjacent limestones at Gap, Limeville 

 and towards Compassville. A few Scolithus borings were 

 noted in the quartzites northeast of Gap, and fragments of 

 Olenellus and specimens of Obolella were found in sandy 

 layers embedded in a shale one mile N.NE. of Gap. The 

 section from the top downward, from a point a little east of 

 Gap northwest, is : — 



5. Massive-bedded, light-colored limestone with partings and 



small, interbedded, flattened nodules of mica schist. 

 4. Narrow belt of hydromica schist, with thin layers of hard 



calcareous sandstone containing Obolella and fragments 



of Olenellus. 

 3. Massive beds of bluish-black and nearly white limestone, 



extensively quarried at Limeville. 

 2. Narrow belt of shale altered to a hydromica schist. 

 1. Quartzite, in the hill northeast of Gap. 



We next visited the extensive quarries at Bellemont, on the 

 main line of the Pennsylvania railroad, four miles west of Gap. 

 In the quarries the conglomerate limestone, so characteristic of 

 the Cambrian at Stoner's station in York county, are beauti- 

 fully exposed, and numerous photographs were taken of the 

 conglomerate beds, showing their positions between the evenly- 

 bedded limestones. The limestones and conglomerates belong 

 to (3) of the Gap section, and are overlain by a belt of shale in 

 which massive beds of a fine, limestone conglomerate occur. 

 No time was given to searching for fossils. 



The discovery of the Olenellus fauna in the limestone in the 

 eastern portion of Lancaster county, north of Gap, taken in 

 connection with the eastern section in York county, compels 

 the reference of the so-called Potsdam rocks of Chester county 

 with their superjacent limestones to the Cambrian. As 

 mentioned in the previous paper, it is quite probable that the 

 limestones towards the Triassic area, in the northern portion 

 of Lancaster county, may be of Ordovician age, but this can 

 only be proven by the discovery of the fauna. 



Northern belt of Limestone. — As shown by the geological 

 map of Pennsylvania (1884), the northern belt of limestone 

 enters the State in Franklin county and then turns to the 

 northeast in Cumberland county, crosses the Susquehanna at 

 Harrisburg and extends on eastward across Dauphin, Lebanon, 

 Berks, Lehigh and Northampton counties, to the Delaware 

 river. The Olenellus fauua was found in the quartzites of 

 South mountain in Adams county, as well as in the lower por- 



