358 Nason — Geological Relations and the Age of 



Art. XXX YII. — The Geological Relations and the Age of the 

 jSt. Joseph and Potosi Limestones of St. Frangois County, 

 Missouri; by Frank S. Nason. 



During the months of March and April, 1901, the writer 

 was engaged in some engineering work lor the Derby Lead 

 Co. of St. Francois county. The nature of the work necessi- 

 tated a study of the local geology. Incidentally this led to 

 the exact determinations of the relations existing between the 

 underlying St. Joseph, or Bonne Terre, and the overlying 

 Potosi, as well as the age of the two limestones. 



The writer is indebted to Mr. O. M. Bilherz, superintendent 

 of the Doe Run mine at Flat river, for first calling his atten- 

 tion to fossils which he had found, and to Mr. Arthur Thacher 

 and Mr. J. T. Morrell, president and superintendent of the 

 Central Lead Co., for the assistance which they tendered him. 



The rocks of St. Francois county have a general, but very 

 slight, S.W. dip. The country is hilly, the higher points 

 reaching from one hundred to one hundred and fifty feet 

 above the datum level of Big and Flat rivers. The hills and 

 ridges are not due, in general, to either monoclinal or anticlinal 

 folds, but, so far as is now known, to erosion entirely. Both 

 Big and Flat rivers have flood plains a mile or more in width, 

 thus cutting through the overlying measures. Into these 

 rivers on either side flow smaller tributary streams which have 

 cut more or less deeply into the long divides, breaking them 

 up into more or less hill-like domes. The gulches formed by 

 these streams are dry and almost wholly denuded of soil, leav- 

 ing the nearly horizontally bedded rocks exposed. As the 

 summits of these hills are approached the mantle of residuary 

 clay becomes thicker and on the summits of many of the hills 

 this clay, filled with drusy, cherty quartz, is often fifty to one 

 hundred feet thick. The limestones capping these hills and 

 divides are more or less cherty, having cavities lined with 

 druses of quartz, locally known as mineral blossom. In many 

 places these same limestones have their jointing and bedding 

 planes covered with the same quartz. In one locality the writer 

 found a bed of sandy rock completely honeycombed with their 

 shell-like druses. The bed was at least one foot in thickness. 

 Immediately above and below at least fifty per cent of the 

 rock was also drusy quartz. Underneath these strata the lime- 

 stone becomes almost entirely free from quartz and in general 

 appearance is hardly to be distinguished from the St. Joseph 

 limestones. The occurrence of cherty or drusy quartz in a 

 limestone has hitherto been the sole means of distinguishing 



