St. Joseph and Potosi Limestones of Missouri. 359 



between the Potosi and the St. Joseph. If the limestone was 

 cherty it has been called Potosi ; if not, St. Joseph, or Bonne 

 Terre. 



Most of the gulches cut by the streams expose the contacts 

 between the cherty and non-cherty limestones. As the cherty 

 limestones show well up the sides of the hills, it has been 

 assumed that only the summits of the hills and divides were 

 Potosi. 



In the geological map of St. Francois County, Mo. (Bull. U. S. 

 G. S. No. 132, " Diss. Lead Ores of S* E. Mo."), Mr. Arthur Wins- 

 low has accepted this erroneous conclusion and further states 

 {ibid., p. 17) : " The rocks of this formation [Potosi] are found 

 principally west and north of the area here treated of [St. Francois 

 Co.] and they occur within it only over the hills. The upper 

 limits of the formation are, therefore, not reached, and it is 

 probable that no sharp line of separation between it and the 

 underlying St. Joseph limestone exists." 



The writer discovered a positive break between the two 

 series in the bed of Flat river about one mile up the river 

 from Elvins, just a little above where the M. P. and B. T. 

 P. P. bridges the stream. Here the river has washed bare a 

 heavy bed of limestone conglomerate. The appearance of the 

 conglomerate is very striking, being composed of flat, round- 

 edged, disks of limestone lying edgewise, as is shown in the 

 accompanying geological column. The conglomerate here 

 appears to be about ten feet in thickness and below the layer 

 of disk-like pebbles, five to six inches thick, it is massive. 



In the railroad cut above the river exposure, the conglomer- 

 ate is seen to be overlaid with soft clay slates from a few 

 inches to several feet in thickness. From this up to the chert- 

 less Potosi, there is a succession of clay slates interstratified with 

 thin beds of conglomerate. Judging by drill holes and by 

 natural sections, the thickness of the conglomerate series is not 

 less than fift} T feet and probably in places it is one hundred 

 and fifty to two hundred feet thick. The conglomerate is not 

 a mere local occurrence. From its outcrop in Flat river the 

 writer traced it continuously for one mile towards the Central 

 Lead Co.'s office ; and in isolated exposures for another mile 

 to near the Theodora shaft of the same company. Southeast, 

 it was found on the face of a bluff about two -miles from Farm- 

 ington, about four miles air line from the original outcrop. 

 To the west, near Irondale, a distance of about eight miles, 

 drill cores showed it to be present at a depth of two hundred 

 and sixty-two feet. 



The pebbles of the conglomerate, as far as seen, are all 

 magnesian limestone. The interstitial paste or cement is with- 



A.M. Jour. Sci.— Fourth Series, Vol. XII, No. 71.— November, 1901. 

 25 



