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logical formation, and a type intermediate between them. One of these 

 lirincipal types, the Knobstone formation, consists of compact, insoluble, 

 impervious sandstones and shales; and the other, the Mississippian lime- 

 stones, consists of conspicuously fissured and jointed, highly soluble, and 

 consequently pervious limestones. It is also apparent that these two prin- 

 cipal types of formation present interesting differences of topography, 

 which are of importance to the student of water-supply problems. 



IT. 



The first of these, the Knobstone formation, consists of a considerable 

 thickness of fine-grained sandstones, with clay cementing material; and 

 of sandy shales, becoming more argillaceous toward the base of the 

 formation. Both sandstones and shales are imi)ervious to an unusual 

 degret?. The evidence of this is seen iu the general absence of springs 

 in the region of the Knobstone formation, in the impossibility of obtaining 

 good wells, either deep or shallow in the rock, and in the small dry- 

 woather flow of the streams in the area underlain by this rock. An 

 indirect evidence ot the minute size of the pores of the Knobstone sand- 

 stones, is the damage that the rock suffers when exposed to freezing. 

 Experiment and microscopical examination reveal the same thing. If a 

 sample of the rock be tested, it will l>e found to absorb water ratlier 

 readily, but to transmit it very slowly. As a matter of fact the purely 

 geological evidence already presented, of the imperviousness of the rock, 

 is altogether more satisfactory than the experimental evidence mentioned, 

 because it deals with the formation in masses commensurate with those 

 with which the water-supply engineer has to deal. 



What the Knobstone formation lacks in water-bearing qualities, it 

 more than makes up iu its perfection as a substratum for reservoirs and 

 ponds. Its qualities in this respect will be brought out in the description 

 of a typical water-supply plant — that belonging to Indiana University — 

 and need not be further discussed at this point. It is sufficient to say 

 here that wherever the conditions are sucli that an ade<iuate supply of 

 pure water can be impounded, the Knobstone formation may be de- 

 pended on, with properly constructed works, to hold the water with a 

 minimum of leakage, and with perfect security to whatever structures are 

 placed upon it. 



The soil cover in the region of the Knobstone is usually ratlier thin, 

 owing to the steepness of the slopes. It is of a sandy character, more 



