594 A. W. GKABAU TYPES OF SEDIMENTARY OVERLAP 



probably owe their color to the presence and innumerable minute spores of 

 Bhizocarps, Protosalvinia huronensis; and the black muds of partly in- 

 closed basins like that of the Black sea are deep-water deposits, where in 

 the denser lower portions of the water H 2 S is generated in great quantity 

 by the activities of sulpho-bacteria.* 



If we now set out to interpret the black shale so characteristic of the 

 mid-Paleozoic of the interior region of North America by the light of the 

 facts gained from a study of modern black mud deposits, we are con- 

 fronted by evidence which points to one or more of the causes cited as 

 probably operative in the production of this deposit. That portions of 

 this shale are due to deposition in a relatively inclosed area, under con- 

 ditions similar to those existing in the Black sea at the present time, 

 seems probable, since some of these shales in the Portage formation of 

 New York are especially rich in iron sulphide, and are further character- 

 ized by the presence of a dwarf fauna, such as is found to be buried in 

 the black muds accumulating in the Black sea today, f But it by no 

 means follows that all the Black shale of eastern United States was depos- 

 ited in this manner; indeed, the evidence does not admit it even as a 

 tentative assumption. The facts are best set forth by a review of the 

 sections in which the Black shale holds a significant position. 



Beginning in the westernmost area of its development on the Missis- 

 sippi, we find a significant series of sections which may form the basis 

 for the interpretation of the southern shale deposits. The following sec- 

 tion was studied by the writer at Louisiana, Missouri, the northwestern- 

 most point of appearance of the so-called Devonian Black shale : 



Section at Louisiana, Missouri 



Feet 



Louisiana limestone. — Compact limestone or calcilutyte resembling litho- 

 graphic limestone 50 



Immediately below this limestone is a bluish gray arenaceous mud rock, 

 resembling the unweathered Chonopectus* sandstone of the Burling- 

 ton section ; when weathered it has all the aspect of that sandstone 1 



In one locality the lower part of this lower bed is more argillaceous, 

 containing a fairly rich Kinderhook fauna, with Spirifer marionensis 

 and Productella concentrica predominating. This shale passes 

 downward without any perceptible break into black fissile rusty 

 shale, resembling in all respects the Genesee shale of New York or 

 the Black shale of Ohio, with which, on this account and on account 

 of its position, it has been identified 4 



* Andumow : La Mer Noir. 



t See Clarke : Naples Fauna, pt. ii, Mem. 6, N. Y. State Museum. Also F. B. Loomis : 

 Kept. State Pal., 1902. 



