476 T. C. HOPKINS — CAMBRO-SILURIAN LIMONITE ORES 



every geological formation and in ever)' county in the state, but this 

 paper treats only of those in the limestone valleys of the central and 

 eastern portions and the closely associated ores of the underlying slates. 

 What is said will apply equally well to many similar deposits along the 

 Great valley of the Appalachians, but it' is not intended primarily to 

 include all of the Great valley deposits, or any part of them outside of 

 Pennsylvania. 



Much has been written about these limonite deposits, and many ex- 

 planations have been offered to account for them. While the different 

 hypotheses heretofore advanced have been taken into consideration, the 

 conclusions in this paper are, as previously stated, based primarily on 

 field observations made by the writer during the years 1898 and 1899. 



Geological Position of the Ores 



The limonite ores in question occur in residual deposits on the Ordo- 

 vician and Cambrian limestones and slates. They are referred to in 

 literature as the Cambro-Silurian ores, signifying that they occur partly 

 in the Cambrian and partly in the Silurian rocks, but that portion ot 

 what was formerly called Silurian is now more commonly called Ordo- 

 vician. The Ordovician portion of the series is included in the number 

 II or Trenton group of the classification of the Second Pennsylvania 

 Geological Survey. Trenton, as thus used, includes all the Ordovician 

 below the Utica shale. The lower portion of the series, including the 

 slates and quartzites, is called group number 1 or " Chiques quartzite " 

 by the Pennsylvania Survey. In the earlier reports it was correlated 

 with the Potsdam of New York, but Lesley in his later reports advised 

 the use of the local term, Chiques or Hellam. Walcott's more recent 

 investigations* show that the quartzite, slates, and some of the lime- 

 stones carry the Olenellus fauna, and hence are Lower Cambrian. The 

 Upper and Middle Cambrian faunas have not been determined in this 

 area, except the lower horizon of the Middle Cambrian in one locality. 

 The upper portion of the limestones carries the Trenton fauna, and a few 

 Chazy and Calciferous forms have been found in lower portions, but the 

 strata between the Trenton at the top and the Lower Cambrian have so 

 far shown no well defined fauna, and have not been carefully classified. 

 It has not been possible even to draw a definite boundary between the 

 Cambrian and the Ordovician, so we are compelled to still speak of the 

 strata in their entirety as Cambro-Ordovician. It is on them that the ores 

 in question occur, some near the top of the Trenton, some on the Lower 

 Cambrian, and many over the vague horizons between these limits. 



* Bull. No. VM of the U. S. Geo!. Survey. 



