606 w. a. nelson volcanic ash bed in the ordovician 



Extent of Ash Deposit 



The southernmost occurrence of the ash deposit thus far known is at 

 Bessemer, Alabama, a suburb of Birmingham, 145 miles south of Single- 

 ton, Tennessee. At this place it is reported to measure from 14 to 18 

 inches thick. The northernmost exposure seen to date is at High Bridge, 

 Kentucky, 190 miles to the north of Singleton, Tennessee, where it meas- 

 ures from 4 to 10 feet thick. This makes the known north and south ex- 

 tension of this ash fall 335 miles. 3 This ash bed may well have extended 

 much farther to the south and north than the limits mentioned, and 

 thinned out gradually as the distance became greater and greater from the 

 volcanic center. Several inches of impure clay have been found 50 miles 

 northwest of Singleton, around Xashville and Franklin, where this ash 

 horizon dips under the surface, on the western side of the Xashville dome. 

 The limits of the bentonite bed undoubtedly extend still farther westward 

 than Franklin. To the east beyond Dayton, Tennessee, the bentonite is 

 undoubtedly now absent, as this area was occupied by a landmass aud tbe 

 ash bed would have subsequently been removed by erosion. To thj north 

 of High Bridge, the point where the thickest bentonite was seen, no work 

 has been done, yet undoubtedly the northern limits of the bentonite must 

 be north of the Ohio Kiver, in middle Ohio and Indiana. The limits of 

 the area covered by this bentonite bed may have been from just south of 

 Lake Erie on the north to the Gulf of Mexico on the south, and from 

 western Virginia and North Carolina on the east to Missouri and Arkan- 

 sas on the west. Such an elliptical area is about 800 miles long by 450 

 miles wide. Before attempting any estimate of the amount of ash de- 

 posited over this area, it is necessary to determine as nearly as possible 

 the conditions under which the ash was deposited and the location of the 

 volcano producing it. 



Description of Bentonite Outcrops 



The bentonite occurs at Singleton, Tennessee, 8 feet below the top of 

 the Lowville limestone of the Black Kiver group, which is overlain by the 

 Hermitage formation of the Trenton group. To the northeast, at Pike- 

 ville, Tennessee, the same section occurs. At Dayton, Tennessee, 15 miles 

 to the east of Pikeville, the section is the same, with the exception that 

 the limestones lying on top of the bentonite have become somewhat shaly 



3 Since this paper was given at Amherst, Dr. Walter A. Bucher, of the University of 

 Cincinnati, has reported to the author the occurrence of this bentonite bed in well num- 

 ber 2, Sinking Spring Oil Company, in the southwest corner of Perry County, Ohio, at a 

 depth of 1,242 feet. This is 110 miles north of the High Bridge, Kentucky, outcrop. 



