316 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



and lower parts, are poorly preserved and rather rare in most localities. Chemically the lime- 

 stone from this part of the formation is foimd to be high in its percentage of magnesiimi. A 

 sample from Dublin yielded 41.07 per cent of magnesium carbonate, while several from Belle- 

 fontaine gave even a higher percentage for this constituent. 



The upper part of the Columbus limestone, which includes about two-thirds of th,e forma- 

 tion, consists, in the main, of light-gray limestone in even beds varying from a few inches to 

 several feet in thickness. At places it contains a considerable amount of white or light-gray 

 fossUiferous chert, which is mainly restricted to a few zones, where it occurs in concretionary 

 masses which are arranged in somewhat definite layers. The upper layers of the limestone are 

 the t hin Ti er and often of a bluish color. The heavy beds, where long exposed, weather into 

 rather thin irregular layers which break into angular blocks and fall to the base of the cliff, 

 forming the usual talus slope. This portion of the formation always shows a crystalline or 

 subcrystalline structure. It is high in its percentage of calcimn carbonate, which ranges from 

 81.14 to 93.28 per cent in samples taken from Marble Cliff quarries. While all layers seem to 

 contain abundant fossils, many are literally crowded with the remains of the various species 

 which swarmed in the waters of this warm Devonian sea. 



The base of the Columbus limestone rests upon the Monroe formation, this contact being 

 that between the two great systems, the Silurian and the Devonian. There is thus a great 

 time gap or unconformity between these two formations, which is strikingly illustrated by the 

 decided change in character and abundance of animal remains. In some localities the lowest 

 layers of the Columbus contain an abundant fauna which in many respects resembles that of 

 the upper part of the same formation, but where these lower layers have been observed in 

 Franklin, Delaware, and Union counties a basal conglomerate is foimd which consists of large 

 and small waterworn pebbles of the imderlying formation embedded in a matrix of Columbus 

 limestone. Where this conglomerate is developed few fossils are found, probably because the 

 organic remains which existed in these localities at the time the layers in question were being 

 deposited were ground to a shapeless mud by the continuous action of the waves among the 

 pebbles of a rocky coast. This conglomerate was formerly supposed to represent the Oriskany 

 sandstone of New York and was so mapped by the geologists who made the- first county reports, 

 as well as by Newberry himself; but since this basal conglomerate is not continuous and has 

 not been proved to be Oriskany, it has been customary of later years, and perhaps wisely, to 

 drop the Oriskany sandstone from the Ohio scale and include these deposits with the Columbus 

 limestone, to which they are at least very closely related. 



The upper limit of the Columbus is no less interesting, siace it terminates in the famous 

 "bone bed" first described by Orton. This layeif or bed comprises the upper 6 or 8 inches of 

 the formation and is frequently made up of an " assemblage of millions on millions of generally 

 imperfect but mostly recognizable organs or fragments of the bony structure of the forms of 

 fish life most characteristic of the Devonian age." These teeth and dermal plates are often in 

 an excellent state of preservation, retaining even their original luster. Contrary to what has 

 usually been supposed, this limiting layer of the formation may be traced northward to San- 

 dusky, where it has proved of invaluable assistance in determining the line of division between 

 the Columbus and Delaware limestones. 



The Delaware Umestone extends from the "bone bed" upward through a thickness of 

 about 36 feet to its contact with the Olentangy shale. It is extremely variable in its appear- 

 ance, consisting sometimes of thin shaly layers, beds of chert, and fairly massive limestone 

 and again almost entirely of rather massive limestone with very little chert and no shale. 

 Usually it has a deep-blue to slate color, which becomes brown on weathering. The chert 

 which it contains is mostly black and nonfossiliferous, but in sections where little occurs it is 

 often a light bluish white or even pure white and somewhat fossiliferous. In general the Dela- 

 ware limestone is less fossiliferous than the upper part of the Columbus, but its fauna is by no 

 means small, and frequently layers are found which are very fossiliferous. The variable chemical 

 composition of this formation is easily seen by a reference to the analysis of various samples 

 collected at different localities. 



