Lesley.] -^(^ [Jan. 2 and Feb. 6, 



The original source of the brown hematite iron ores of our Lower 

 Silurian limestone valleys has been speculatively sought for vrithout 

 sufficient investigation in the field ; and much practical mischief has re- 

 sulted from the errors promulgated. Most persons have looked upon 

 them as accidental and local inwashes from unknown sites. Some have 

 more systematically defined them as a residual precipitate from the dis- 

 seminated iron-sand grains of the surrounding Middle Silurian mountain 

 rocks during their erosion. 



All such vague speculations might have been avoided had the results of 

 Dr. R. M. S. Jackson's survey of the Nittany Valley ore beds in 1838 or 

 1839 been published by himself. As assistant on the geological survey ot 

 Pennsylvania he obtained the data necessary for concluding, at that early 

 »day, that they were deposits in loco originali, of the iron (as hydrated 

 ;peroxide) set free from the limestone or dolomite rocks during their 

 :gradual erosion and dissolution. 



I have myself, during the last twenty years, had ample opportunities 

 for arriving independently at the same conclusion ; and an intelligent 

 study and comparison of the aspects of the ores and rocks in our iron ore 

 banks will, I think, satisfy any good geologist in the same sense. 



The precise modtos operandi of the process is not yet well understood ; 

 for it involves chemical considerations not thoroughly worked out. But 

 a general statement of the operation can be made without I'isk of serious 

 error. 



The rocks of the Lower Silurian Age were originally sea-muds, com- 

 posed of rounded grains of dolomite (derived fi-om previously existing 

 Laurentian Land), cemented togeiher with a paste of carbonate of lime* 

 Some of the beds consisted also of rounded grains of quartz. Some of 

 the layers were nearly pure carbonate of lime. All contained a larger or 

 smaller percentage of iron, lead, .zinc and other metals, precipitated 

 either chemically, or by the agency of organic beings, from the solutions 

 of their carbonates, chlorides, &c., in the river-and sea-waters. The 

 orderly explanation of all the chemical and organic features of this 

 complicated operation is still to be given to the scientific world. But all 

 will agree that the general character of the calcareo-ferruginous muds, 

 the sediments of that early geological age must have been much as 

 above desciibed. 



During the long Upper Silurian, Devonian and Carbiniferous Ages, 

 these Lower Silurian sediments were buried to a depth of over 16,000 feet, 

 beneath the later sediments. They remained wet. Their great depth 

 raised their temperature 16,000 ^- 50 = 320° Farenheit's thermometer ; 

 which added to the mean temperature of the surface, would keep them 

 under the influence of a moist heat of nearly 400° P. through what to 

 man is a small eternity of time. 



Dr. Genth's discovery of the amorphous or gelatinous condition of a 

 part of their silica is thus explainable. Varied reactions must have 

 ensued. The carbonates of lime and magnesia combined as dolomites, 



