12 GEOLOGY OF OHIO. 



but in the great majority of wells, especially in Hancock and Wood 

 counties, it is a constant element, ranging from five to thirty feet. The 

 gas wells are often cased in this shale, but a risk is always taken in doing 

 so, as water is liable to be found in the underlying Clinton rocks. 



In Montgomery, Miami and Greene counties the shale contains, in 

 places, a very valuable building stone, which is widely known as the 

 Dayton stone. It is a highly crystalline, compact and strong stone, 

 lying in even beds of various thickness, and is in every way adapted to 

 the highest architectural uses. It carries about ninety-two per cent, of 

 carbonate of lime. The Niagara shale, as a rule, is quite poor in fossils. 

 It is apparently destitute of them in many of its exposures ; but there 

 are still parts of the state in which it contains a considerable fauna. 

 The best phases of it, in this point of view, are found in Highland 

 county, south of Hillsboro. 



The limestone that succeeds the shale is an even-bedded, blue or 

 drab -magnesian stone, well adapted at many points to quarrying pur- 

 poses. It contains many characteristic fossils of Niagara age. It is 

 known in Ohio by various local names, derived from the points where it 

 is quarried. There are several subdivisions of it that are unequally 

 developed in different portions of the state. Like the shale below it, 

 this member is thickest in southern Ohio. It cannot be recognized as a 

 distinct element in the northern part of the state, either in outcrop or in 

 drillings. It may be that its horizon is not reached in any natural ex- 

 posures of the formation in this part of the state. 



The uppermost division of the formation is the Guelph limestone, 

 which differs very noticeably in several points from the Niagara lime- 

 stone proper. It obtains its name from a locality in Canada, where it 

 was first studied and described. It has a maximum thickness in south- 

 ern Ohio of two hundred feet. It differs from the underlying limestone 

 in structure, composition and fossils. It is either massive or very thin- 

 bedded, rarely furnishing a building stone. It is porous to an unusual 

 extent. It is generally very light in color, and is, everywhere in the 

 state, nearly a typical dolomite in composition. It yields lime of great 

 excellence for the mason's use. It is exceedingly rich in iossils, contain- 

 ing a large number that is thoroughly characteristic. 



Unlike the previously named divisions of the Niagara, the Guelph 

 limestone is as well developed in northern as in southern Ohio in all 

 respects. Not more than forty feet of it are found in its outcrops there, 

 but the drill has shown several times this amount of Niagara limestone, 

 without giving us, however, the data needed for referring the beds trav- 

 ersed to their proper subdivisions. What facts there are seem to point 

 to the Guelph as the main element in this underground development of 

 this formation in this portion of the state. 



The Hillsboro sandstone is the last element in the Niagara group. 

 It is found in but few localities, and its reference to the Niagara series in 

 its entirety is not beyond question. In Highland county it has a thick- 



