KOCKS OF THE KEGION. 285 



certain peculiarities in the distribution of the clay cap, we will conclude 

 that the Topton divide must have been the lowest of the three at the 

 close of glaciation. 



ROCKS OF THE REGION. 



As these have been fully described in the state surveys, only a com- 

 parison of similar features of different formations will be attempted. 

 The Archean rock is a granulite* varying to gneiss, generally syenitic. 

 The quartzite is placed in the Potsdam by the state survey, and varies to 

 a gritty sandstone eastward from Reading and is a red shale locally at 

 South Bethlehem. Its lower |)art is dense arkose, and the localities 

 where the measures are flexed are noted for breccias. The gritty varie- 

 ties abound in Scolithus linearis ; are weathered to a whitish sand, and lose 

 the conchoidal fracture of the quartzite. The pebbles of this formation of 

 a recent origin can be generally told from the Paleozoic pebbles of the same 

 (as found in the basal conglomerate of the Trias) by the reddish or purplish 

 color of the latter, due to long immersion in ferruginous mud. This 

 color extends from a few lines to an inch from the surface, while the in- 

 terior is fresh. There are, however. Paleozoic pebbles which have escaped 

 this staining, and such cannot be told from recent ones, and resemble, 

 also, the rolled pieces of the lighter bands of the Oneida-Medina forma- 

 tion at the Blue ridge. When the latter conglomerate carries slate and 

 chert pebbles there is a ready means of distinction. The Oneida sand- 

 stones have whitish streaks — they cannot be called veins — several inches 

 in length that are not found in the Potsdam rock, but when they do not 

 exist there is difficulty if not impossibility of distinction by the pocket 

 lens, as both have a conchoidal fracture, a similar arrangement of grains 

 and matrix, and a similar habit of weathering. The darker bands of the 

 Oneida-Medina show specks of ocher on a fractured surface. These 

 similarities often make it difficult to base arguments on the })resence of 

 such rocks over portions of the region, as the gneiss was universally over- 

 laid by Potsdam west of the Saucon, and large areas of this formation 

 still exist on the summits of the highlands in the position of deposition. 

 The presence of rounded or angular fragments of sandstones and quartz- 

 ites on gneiss may therefore be residual and not due to transportation, 

 and their presence in abundance in the basal conglomerate of the Trias 

 prohibits their use over that formation, as its limestone pebbles weather to 

 a red clay and the rotten rock simulates some varieties of till, especially 

 where the pebbles have been unstained. - 



The state survey reports f that the limestone belt is bounded above 



* Report of Progress, Second Geol. Survey of Pennsylvania, D 3, vol. i, p. 72. 



fSee Report of Progress, Second Geol. Survey of Pennsylvania, vol. DD, chapter xvii. 



