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

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

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

 stones, consists of consi)icuously fissured and jointed, highly soluble, and 

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

 cipal tyi^es of formation present interesting differences of topography, 

 which are of iniiH)rtance to the student (if water-supply problems. 



II. 



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

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

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

 formation. Roth sandstones and shales are imi>ervious to an uuusnal 

 degree. The evidence of this is seen in 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 I'ock, and in the small dry- 

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

 indirect evidence of 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 be found to absorb water rather 

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

 geological evidence already presented, of the impeiwiousness 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 in 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 siillicient to say 

 here that wherever the conditions are such that an adtMjuate supply of 

 pure water can be impounded, the Knobstone formation niaj^ be de- 

 pende<l on, with iiroperly constructed works, to hold the water with a 

 minimum of leakage, and with i)erfect .security to whatever structui'es are 

 idaced upon it. 



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

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



