GEOLOGICAL SCALE AND STRUCTURE" 35 



under ground than the conditions here found, and consequently the 

 records of deep drillings in southern Ohio become almost as clear and 

 legible as if the rocks through which the drill has passed lay exposed to 

 the light of day. 



The real, though not the formal separation of this group from the 

 underlying shale is due to the late Professor E. B. Andrews, and consti- 

 tutes one of his most important contributions to our knowledge of Ohio 

 geology. He was the first to show that the great conglomerate of Hock- 

 ing, Fairfield and Licking counties is Sub-carboniferous in age, and he 

 further called attention to a highly fossiliferous, fine-grained sandstone 

 overlying the conglomerate, to which he gave the name of Logan sand- 

 stone, from its occurrence at Logan, Hocking county. Up to this time 

 this conglomerate had been universally counted as the Coal Measure con- 

 glomerate. Read made know the existence of a heavy body of shale, 

 which he called Olive shales, overlying the conglomerate and replacing 

 the Logan sandstone in Knox, Holmes and Richland counties. 



As both conglomerate and sandstone have their typical outcrops at 

 Logan, no better name can be found for the formation, which must 

 include conglomerate, sandstone and shale, than that here adopted, viz., 

 the Logan group. 



The maximum thickness of the Logan group is not less than four 

 hundred feet. Its average thickness is perhaps two hundred feet. It has 

 received less study than the rest of the series, and it is only within the 

 last three years that divisions have been recognized in it by means of 

 which a measure of order can be given to its principal outcrops. 



12. The Sub-carboniferous Limestone. 



This element is of comparatively small account as far as its surface 

 outcrops are concerned, in Ohio, but it gathers strength to the southeast- 

 ward, and is shown in several well records of the Ohio Valley, in the 

 eastern part of the state, as a stratum fifty or more feet in thickness. It 

 was recognized as a member of our geological column by the geologists 

 of the first survey, but Professor Andrews was the first to assign to it its 

 proper place, and to show its true equivalence. He designated it the 

 Maxville limestone, from a locality in southwestern Perry county, where 

 it is well exposed in beds that aggregate fifteen or twenty feet in thick- 

 ness. Still heavier deposits of it are found in the valley of Jonathan's 

 Creek, in Muskingum county, near Newtonville. Professor Andrews 

 collected at these points the fossils by which its age was determined to 

 be that of the Chester limestone of the Missouri and Illinois sections. 



In its best development, the limestone is a fairly pure, fine-grained, 

 sparingly fossiliferous rock. It breaks with a conchoidal fracture. In 

 fineness and homogenity of grain it approaches lithographic stone and 

 has been tested practicalby, in a small way, for this use. It is seldom even 

 or regular in its bedding. It is light drab or brown in color, and often is 

 a beautiful building stone, though somewhat costly to work. 



