CAMBRIAN AND LOWEE ORDOVICIAN. HI 



The Beekmantown is a rather pure limestone lying between the siliceous Conococheague 

 below and the very pure Stones River above. A minutely laminated appearance on weathered 

 surfaces of many of the beds, due to their impurities, and pink to white fine-grained limestone 

 or marble are characteristic features of the formation. Near the base are sUiceous banded 

 beds and large "edgewise" conglomerate, closely resembling the Conococheague formation. 

 These have been separated as a transition phase under the name Stonehenge member of the 

 Beekmantown. 



Although sparingly fossiliferous as a whole, the Beekmantown has yielded 

 a large variety of forms, which are listed in Stose's paper and in accordance with 

 which the Pennsylvania occurrence is correlated by Uhich with the New York 

 formation of the same name. 



For descriptions of the overlying later Ordovician limestones see Chapter IV 

 (pp. 176-179). 



The Cambrian and Ordovician limestones are exposed in Center County, Pa., 

 on the Nittany anticline, where Uhich has distinguished several subdivisions of 

 the Beekmantown. 



In eastern Pennsylvania, in the vicinity of Philadelphia, the early Paleozoic 

 rocks are highly metamorphosed. The Wissahickon mica gneiss, which has been 

 considered to be of Cambrian or Ordovician age, is now regarded as pre-Cambrian. 

 Its relations to the Paleozoic are clearly described by Bascom.'** The Cambrian 

 is represented by the Chickies quartzite and part of the Shenandoah limestone. 

 The upper part of the Shenandoah is of Lower Ordovician age, probably Beek- 

 mantown. The middle and possibly later Ordovician are represented by the 

 Octoraro schist, in some part at least. (See pp. 177-178.) 



Regarding the Cambrian and Lower Ordovician (Chickies quartzite and 

 Shenandoah limestone), Bascom says: 



The Chickies quartzite usually shows a conglomeratic lower member, which is largely 

 composed of elongated pebbles of the blue quartz that characterizes the pegmatites and 

 some facies of the Baltimore gneiss. This lower member of the quartzite is not often exposed. 

 * * * The conglomerate passes upward into a gray, compact, crystalline quartzite, which, 

 in turn, grades into a siliceous slate or a sericitic quartz schist, or is altogether supplanted by 

 the quartz schist. * * * 



Thirty miles west of the Philadelphia district a section through the Chickies quartzite 

 shows a considerable thickness of micaceous feldspathic material interbedded with the quartzite. 

 Such beds occur as the uppermost member of the series, separating typical quartzite from 

 the overljdng limestone, and also as a lower member. Toward the east these micaceous beds 

 become very thin and are altogether absent from the Philadelphia district. * * * 



The thickness of the formation varies; it never exceeds and is often less than 1,300 feet, 

 although the isoclinal folding in some localities gives the appearance of greater thickness. An 

 overturned synclinorium with stratification and cleavage dips to the southeast is the prevailing 

 structure. * * * 



The name of the formation is taken from the locality of its fijaest exposure and greatest 

 thickness, on Susquehanna River north of Columbia. At this locality the quartzite shows 

 abundant traces of Scoliihus linearis, as is the case also in the North Valley HiUs, and underlies 

 quartzite in which Olenellus fragments have been found by Walcott, thus establishing its age 

 as Georgian ("Lower Cambrian"). The quartzite of the Philadelphia district deposited farther 

 to the east than this typical exposure of Georgian quartzite on Susquehanna River may have 

 been laid down in an encroaching sea and thus belong to a later stage in the Cambrian than the 

 Georgian. No forms of life save Scolithus linearis have been found in it, hence it can not posi- 



