286 PROCEEDINGS OF THE ACADEMY OP [1880. 



be desirable brieflj^ to sketch the geolog}', and especially the 

 surfoce geolog}' of the valley in which it occurs. 



The underlying rock is an altered lower Silurian limestone, the 

 "Auroral " of Rogers, which in the southern part of the valley is 

 crystalline marble and in the northern part is a sand}^ magnesian 

 limestone. It has an E. and W. strike and a steep south dip, and 

 is supposed to have an inverted synclinal structure. The lime- 

 stone rises to the surface in a series of parallel ridges, and between 

 these lie the iron ores and the lignitic strata. Bounding the valley 

 are hills of altered shale of probably Potsdam age. Beyond this 

 line of hills to the north, are the Triassic red shales and sand- 

 stones, while to the south is the Philadelphia gneissic district. 

 In many places the North Valley Hill has been eroded away and 

 Triassic strata lie directly upon the limestone. 



The iron ores of this region probably belong to four different 

 geological ages, and may therefore be divided into four classes. 



1. Gneissic Ore. This ore, never found in the valle3^, occurs in 

 the gneissic rocks of Chester County north of the Chester Vallej^, 

 and has been formed in place from the altered gneiss. It dips 

 with the gneiss, and is generally accompanied by scales of graphite. 

 Prof. Rogers^ supposed that this ore belonged to isolated patches 

 of Triassic red sandstone. The writer, however, has not been able 

 to confirm his sections, nor to show the presence of any more 

 recent formation than the aneiss. 



2. Primal Ore. The hydromica slates which lie between the 

 Potsdam sandstone and the limestone liberate, when decomposed, 

 a rich limonite ore which is largely mined in jjortions of the 

 valley. Although in very irregular beds, a steep dip can be re- 

 cognized. It is perhaps derived from the decomposition of pyrite. 

 This is pi-obably the ore mined at Edge Hill. 



3. Tertiary Ore. This ore, associated with which are the de- 

 posits of lignite, plastic clay, kaolin, fire-sand, etc., has been 

 hitherto confounded either with the Primal ore or Avith the Drift 

 ore of the valle^'. In that part of the valley under discussion 

 there are three distinct lines of outcrop of this ore, having nearly 

 an E. and W. trend. A ridge of limestone separates two of these 

 lines. The ore lies, sometimes at a great depth, below a re-stratified 

 decomposed h3dromica slate. This latter formation is almost 

 identical in appearance with the decomposed Primal slate in place 



1 Geol. of Penna., I, 87. 



