484 T. C. HOPKINS — CAMBRO-SILURIAN LIMONITB ORES 



age of silica, and from dolomite to a variety nearly free from magnesia. 

 In Nittany valley, in Center county, the upper portion of the limestone 

 is highly calcareous, while the lower portion is very arenaceous, in fact 

 grading in several places into a silicious sandstone, which reaches a 

 thickness of 300 or 400 feet. 



White Clay 



Intercalated with both the Ordovician and the Cambrian limestones 

 and associated with the Cambrian quartzites underneath the limestones 

 are beds of hydromica slates, which on exposure weather into clays. 

 These clays are frequently parti-colored from the iron oxide stains, but 

 in many localities are almost entirely free from iron and are nearly snow 

 white. These white and parti-colored clays are very intimately associ- 

 ated with the iron ores, in a great many banks the ores resting on or 

 against a deposit of the clay. 



Exposures of the white clay are most numerous in the South Moun- 

 tain area, in Cumberland county, where it occurs in nearly every one of 

 the ore pits designated on figure 5. Similar deposits occur at various 

 horizons in the limestone valle} 7 s, but most abundantly near the base of 

 the Cambrian portion of the limestones. In Lehigh and Berks counties 

 there are several ore pits exposing white clay in the limestones near the 

 top of the Trenton, not far from the contact with the overlying shales. 

 In Nittany valley the clays occur in the proximity of the sandstone 

 beds in what probably corresponds to the Calciferous division, as Calcif- 

 erous fossils were found in what is to all appearances the same horizon. 



The evidence that the white clays are the weathered products of the 

 hydromica slates is (1) the intimate gradation of the clay into the 

 slightly weathered slates, which is shown clearly in clay pits at Latimore, 

 York county, and in the ore pit at Hensingerville (see figure 4) ; (2) the 

 occurrence in the clay of several ore pits of fragments of the undecom- 

 posed slate, and (3) the occurrence in many of the cla} r exposures of 

 numerous fragments of white quartzite, similar in appearance to the inter- 

 laminated thin quartzite lenses which are nearly always present in the 

 slate exposures. 



In the Lower Cambrian horizon below the limestones are beds of sand- 

 stones, quartzites, conglomerates, and talcose slates. There are many 

 diabase dikes cutting through the sedimentary rocks in different local- 

 ities, but they bear no direct relation to the ore deposits. 



Mode of Occurrence 



The ores occur for the most part in fragments of varying sizes, com- 

 mingled with the residual material resting upon limestones, slates, or 



