248 PROCEEDINGS OP THE ACADEMY OF [1884. , 



Pennsylvania from New Jersey. In New Jersey they are iden- 

 tified as belonging to the Laurentian." 



With these points in view, I have followed Mr. Hall, and placed 

 the syenitic rocks of Delaware in the Laurentian, although I feel 

 that it is, at best, but a problematical designation. Any positive 

 declaration upon this point would be premature, until a more 

 thorough structural and petrographical study has been finished, 

 together with a comparison of results from diverse localities. 

 Throughout southeastern Pennsj^lvania the hornblendic rocks are 

 always stratigraphicall}'- the lowest, and such is the case in 

 northern Delaware. 



The general dip of the syenitic rocks, upon the flanks of which 

 rest the strata of the micaceous belt, is, in the latter locality, to 

 the northwest. In the western part of the State, however, the 

 hornblendic rocks have experienced an overthrow, whereby they 

 dip to the southeast, in which case the hornblendic rocks are 

 apparently the younger. This peculiarity need not, however, be 

 misleading as to true stratigraphical order. Putting aside all 

 questions of position of strata, one must note the decidedly 

 primitive aspect of these rocks, which, in lithological characters, 

 are identical with the rocks of more northern portions of the 

 Laurentian area. 



The Potsdam. — In the northwestern part of Mill Creek Hundred, 

 a triangular area of Potsdam sandstone is seen upon the map, 

 which, rising from beneath a patch of Brj^n Mawr gravel, extends 

 into Pennsjdvania, and is best exposed beyond the State line. At 

 Kivin's limestone quarry, a mass of quartzite forms what is clearly 

 an anticlinal fold, over which is a corresponding anticlinal of 

 magnesian limestone. The anticlinal structure of the quartzite is 

 further shown at a few other points, where dips both to the north- 

 west and southeast are noted. 



The Galciferous — Mr. Hall divided the limestone of south- 

 eastern Pennsylvania into two groups, namely, the Galciferous 

 magnesia limestones and marbles, and the possible Trenton lime- 

 stones and slates, the former comprising the rocks of the Chester 

 County limestone valley, and several outlying troughs to the 

 south, the latter those alternations of slate and limestone which 

 form the outer border of the Calciferous belt. 



The limestone areas of Delaware belong to the lower of the 

 above groups, or to the Calciferous of Mr. Hall, the equivalent of 



