48 



Over much of the northern part of its main area m Indiana, the 

 Knobstone is at present more or less covered by glacial debris, its strata 

 being exposed only in the stream valleys. The shales of the basal or 

 eastern third of its unglaciated portion are excellently adapted to the 

 making of vitrified wares, as paving brick, sewer pipe, etc.. as well 

 as for the clay ingredient of Portland cement: though as yet their pos- 

 sibilities of service for these products have been largely ignored. 



Following the Knobstone epoch came that of the Lower Carbonifer- 

 ous limestones. Four distinct hoi-izons of these limestones are recog- 

 nized in Indiana, viz., the Harrodsburgh. Bedford, Mitchell and Huron, 

 in the order named; each representing a distinct period of deposition 

 in the slowly retreating Central Interior Sea. Their total thickness is 

 nearly 600 feet, and together they form the surface i-(.cks over an area 

 40 miles wide on the Ohio River, but which gradually narrows north- 

 ward until it disappears beneath the drift in the vicinity of Crawfords- 

 yille. Montgomery County. 



Of the four horizons that of the Bedford is by far the most noted, 

 since from it is obtained tliat famous Bedford or Indiana oolitic lime- 

 stone which is now widely recognized as the finest l)uilding stone on 

 the continent of America. It is mainly composed of the globular sliells 

 of microscopic foraminifera or Rhizopods— minute one-celled animal or- 

 ganisms—which must have swarmed in untold myriads in the sea waters 

 of the time. The shells or cell walls of these animals were composed 

 of a very pure carbonate of lime, and when they died :ind sank on 

 the old sea bottom these shells were cemented together by the same 

 material. Under the lens they resemble a mass of fish eggs soldered 

 together, hence the name "oolitic," meaning "like an egg." The Bedford 

 stone Is noted among architects for its strength and dural)ility. and 

 for the ease with which it may be sawed or carved into any desired 

 form. For many years it has ranked as one of the principal natural 

 resources of the State. 



The "Mitchell limestone" overlying the oolitic is composed of a series 

 of close-grained limestones, shales and cherts. Its outcrop— 5 to 30 

 miles in width— is a fairly level plateau which is pitted witli a great 

 number of sink holes, many of which form the openings into under- 

 ground caverns and the beds of subterranean streams. The thick lieds 

 of Mitchell limestone, taken in connection with the und(>rlying Bedford 

 and Harrodsburgh limestones, afford a series of rocks which are more 



