ORIGINAL SOURCE OP THE ORES 491 



Each of them has furnished a portion of the iron for the ore deposits, 

 but not necessarily all in the same ratio or even in proportion to the 

 amount of iron which they contain. The manner in which erosion 

 removes material has much to do with what becomes of the different 

 constituents. The erosion of sandstones, clays, and slates is largely 

 mechanical, and the included minerals would in large measure be carried 

 away as sediment along with the rock material. Limestone, however, is 

 largely removed in solution, and the insoluble constituents accumulate 

 in the residue. This is one of the reasons why the writer thinks that 

 much of the iron ore of the limestone valleys has been derived from 

 the Cambro-Ordovician limestones on which they occur. 



The reasons for attributing the source of some of the iron to the over- 

 lying sandstones and slates are (1) that these rocks contain iron and 

 overlie the rocks on which the ores occur; (2) in some places meteoric 

 waters pass through or over these rocks into the limestones, and as the 

 waters contain organic acids, they would naturally dissolve some of the 

 iron ; (3) in some places seepage from the shales at the top of the lime- 

 stone has been observed to be impregnated with iron. The reasons for 

 thinking that this source of supply is not large in comparison with that 

 from the limestones are (1) that the shales and slates are not readily 

 permeable and serve to turn the water from the limestone where topo- 

 graphic conditions make it possible ; (2) that in general the most exten- 

 sive ore deposits are near the base of the limestones ; (3) and that the 

 erosion of the shales is largely mechanical and rapid and most of the 

 iron contained in them would be carried away as sediment. 



That the iron in the ore deposits on the lower horizons has been derived 

 in part from the Lower Cambrian slates seems probable, in view of the 

 fact that the slates in some places are impregnated with iron pyrites, the 

 weathered surface often showing many minute cubical cavities from 

 which the pyrite has been leached out. The slates intercalated in the 

 limestones are likewise a probable source of iron, as analyses of samples 

 from three different localities show the following percentages of ferric 

 oxide : 0.91, 5.06, and 2.40. There is a possibility, however, that part of 

 the iron in the slates analyzed may have been carried in from the over- 

 lying limestone. That all of the iron is not derived from the hydromica 

 slates is evident from the fact that extensive deposits of ore occur in the 

 Ordovician limestones in localities where the slates do not occur, except 

 in horizons below the ore deposits. The slates, as noted elsewhere, occur 

 in the Ordovician limestones in Lehigh county, but they are not present 

 in Nittany valley, except near the base of the series, in the oldest rocks 

 exposed in the valley. Furthermore, where the slates do occur, the ores 



LXX— Bui,t.. Geot,. Soc. Am., Vot,. 11, 1899 



