46 cS REPORT OF NATIONAL MUSEUM, 1904. 



to him to be reasonable grounds, that the soil itself originated in form 

 of an almost impalpable sediment, which gradually accumulated on 

 the bottom of an immense lake which once occupied the region. 



Hall, as already noted, was assisted in his work on the geological 

 •survey of Iowa by J. D. Whitney, who served as chemist and miner- 

 alogist. A good deal of attention was given by Whitney to the chem- 

 ical nature of the coals and the distribution and mode 

 i^wa ,T8S8 Work '" °f occurrence of the lead and zinc ores. There was, 

 however, very little of a theoretical nature, and specu- 

 lations regarding the origin of the ores were almost entirely lacking. 

 He regarded the iron ore of Jackson as having originated, in some 

 instances at least, through the decomposition of nodules of iron pyrites 

 which are distributed irregularly through the rock. In other cases he 

 thought it to be a deposit from springs, the original material, how- 

 ever, having been derived from the decomposition of pyrite, as before. 



Considerable attention was naturally given to the lead and zinc ores. 

 He noted that where the Galena limestone had a maximum thickness, 

 the lead deposits were limited to the central and lower portions of the 

 rock, and never penetrated the Blue limestone, and he had satisfied 

 himself that, in a great majority of cases at least, the deposits diminished 

 in productiveness rapidly after passing below a limit perhaps fifty 

 feet above the base of the Galena. When, however, the Galena lime- 

 stone was diminished in thickness, he noted that the lead deposits were 

 found in lower and lower positions until, at last, the bottom of the 

 Blue limestone was reached, where the Upper sandstone entirely cut 

 oil' the ore, there having never been a single instance, so far as he had 

 ascertained, in which a crevice had been worked in that rock. 



He found reason, therefore, to differ somewhat with Owen in his 

 statement to the effect that when a mine was sunk through the Cliff 

 to the Blue limestone beneath, the lodes shrank to insignificance, as 

 considerable deposits of ore had been worked in the Blue limestone, 

 though by far the larger portion of the lead of the upper mines came 

 from the lead-bearing rock. 



As to the possibility of the lead-bearing crevices extending down- 

 ward indefinitely and the probability of deep mining ever being likely 

 to prove profitable, Whitne}^ wrote, "The question can be answered 

 unhesitatingly in the negative, 1 ' and went on to state that the most 

 profitable method of mining will be in the form of horizontal excava- 

 tions or drifts and not by means of vertical shafts. 



The Iowa lead deposits, it should be noted, he regarded as occurring 

 in gash veins. The mineral deposits of the Northwest in general were 

 classified under the heads of surface deposits, vertical crevices, and flat 

 sheets, as in his work on the metallic wealth of the United States 

 (p. 438). 



Perhaps the most peculiar idea advanced b}^ Whitney in any part 

 of his report is that regarding the origin of the siliceous matter of the 

 Potsdam sandstone. 



