492 T. C. HOPKINS — CAMBRO-SILITRIAN LIMOXITE ORES 



are in nearly all cases on top of them and not underneath. Unless we 

 suppose the iron to be carried upward by ascending waters, which is 

 hardly probable, there appears to be no reasonable way in which the 

 ores in the upper horizons could be derived from slates in the lower ones. 



The reasons for thinking that the Cambro-Ordovician limestone series 

 is the principal source of the iron in the ore deposits are: (1) the great 

 number of the deposits in the limestone area and their wide distribution 

 over all the different limestone horizons, from the top to the bottom of 

 the series; (2) the intimate commingling of the ores with the residual 

 clay and chert fragments from the dissolution of the limestones ; (3) the 

 almost universal occurrence of the deposits on and not under the inter- 

 calary beds of clay ; (4) the manner of erosion of the limestone, which is 

 wholly by solution, thus offering the most favorable condition for the 

 preservation and the accumulation of the ores; (5) the limestones con- 

 tain iron — in small quantities, it is true, but I think sufficiently large. 

 The accompanying analyses of limestones from different localities show 

 an average content of one and a quarter per cent of iron carbonate and 

 sulphide. A thickness of 6,000 feet of limestone, which is present in the 

 Nittany valley, would represent 40 feet of iron ore, which is probably in 

 excess of any deposit in the valle} r . However, this is subject to a num- 

 ber of qualifications which seriously modify any computations in this 

 line. 



On the one hand it can not be assumed (a) that all the iron in the lime- 

 stone is changed to ore proper, as often a considerable portion of it re- 

 mains in the residual clay in the form of yellow ocher, or (b) that all the 

 ore remains in the residual material, as part of it, although a compara- 

 tively small part, is carried away as sediment in the streams ; I say a 

 small part, because the streams in the limestone area are few in number, 

 small in size, and carry very little sediment ; (c) many of the ore deposits 

 are not at the bottom, but some of them are near the top of the limestone. 



On the other hand, it is to be noted (a) that the limestones are in nearly 

 all cases highly inclined, sometimes almost vertical, and the thickness of 

 limestone eroded over any given ore deposit may be much greater than 

 that represented by a section normal to the bedding at that point. This 

 is illustrated in figure 7, where A would represent the actual thickness 

 of limestone over the position of the ore deposit in normal position, but 

 B represents the actual thickness eroded in the inclined position. The 

 fact that part of the limestone was probably eroded during the process of 

 folding would not materially alter the result, as the residual materials 

 would not be greatly shifted thereb}^. (b) There would be a lateral as 

 well as a vertical segregation of the iron. It has been stated that the ore 



