Chattanooga Shale in Kentucky. 121 



hands of different men. Through the deductive method based 

 on the faunas alone one geologist may find in the total absence 

 of a fauna or faunas evidence of land conditions during the 

 interval represented by the missing fauna ; another with dif- 

 ferent predilections may interpret this absence, if no evidence 

 of subaerial erosion has been adduced, to marine scour or tem- 

 porary suspension of sedimentation without land conditions. 

 Geologists agree that unconformities mark important datum 

 planes in stratigraphic geology. Hence the evidence for them 

 is of sufficient importance to warrant full and complete presen- 

 tation from both the biological and physical points of view. 



In this particular case the faunal evidence affords sufficient 

 proof of unconformity independent of physical evidence, but 

 in various other cases unconformities have been introduced by 

 geologists where theoretic considerations regarding supposed 

 diastrophic movements appeared to require their presence. An 

 example of this kind of unconformity is one recently placed 

 within the New Albany shale* of Indiana (Chattanooga) with- 

 out a vestige of direct evidence in support of it. The history 

 of such an unconformity as this hypothetical one within the 

 New Albany is likely to be similar to that of the unconformity 

 which was drawn at the top of the Chattanooga in central Ten- 

 nessee some years ago by Hayes and Ulrich. f In a recent 

 paper by the last named author;}; this unconformity is aban- 

 doned without any explanation of the nature of the defects in 

 the original evidence. It appears to have migrated to the top 

 of the next formation above the Chattanooga, designated by 

 Ulrich the Kidgetop. Whatever the ultimate verdict regard- 

 ing the unconformity at the top of the Chattanooga shale may 

 be ; the following discussion will show that the unconformity at 

 the base of this formation does not belong to the evanescent 

 class of unconformities. 



Distribution of the Chattanooga Shale. — The Chattanooga 

 shale, briefly characterized, is a formation composed chiefly of 

 black fissile carbonaceous shale ranging from about 240 feet in 

 the northern part of the state to 25 feet in the southern part. 

 The areal distribution of the Chattanooga shale as an outcrop- 

 ping formation in Kentucky is confined to two geographically 

 separable areas. By far the larger and more important of these 

 is the narrow semicircular band which borders the dome of 

 Ordovician and Silurian rocks in Northern Kentucky on the 

 east, south, and west. From Vanceburg on the east to Louis- 

 ville on the west the Chattanooga shale extends around the 



* Ulrich, E. O., Revision of the Paleozoic System, Bull. Geol. Soc. Amer- 

 ica, vol. xxii, pi. 28, 1911. 



f Folio U. S. Geol. Survey, No. 95, 1903. 



{Bull. Geol. Soc. America, vol. xxii, pi. 29, 1911. 



Am. Jour. Sci. — Fourth Series, Vol. XXXIII, No. 194. — February, 1912. 



