122 



Central Kailway, and again in tlio exi-avatlon for the fonndation of the 

 Indiana University dam. When, however, tlie roclv is exposed to the 

 action of the sun in summer, and of frost in winter, the differential expan- 

 sion and contraction in the one case, and the wedging effect of tlie freezing 

 of interstitial water in the other, rapidly reduce the rork to a mass. of 

 fragments, which in turn slack down to a sandy soil. For this reason 

 the sandstone is of no account as a building stone. The peculiarities of the 

 rock, just enumerated, are due in large measure to the fineness of the 

 grain, and to the fact that the cementing material is clay, which, when 

 moist, gives the rock its unique toughness and impermealiility. 



Structurally also this sandstone is extremely favdrahle as a sub- 

 stratum for dams. It is singularly free from open joints and bedding 

 planes. In the case of the Fniverslty dam. which is IKiJ feet long at the 

 base and M4 feet high above the rock, there is nut a single .Joint or 

 bedding seam in the rock except iiear the top. The Imttom and ends of 

 the dam are in perfectly sound and unlissured rock. The thickness of 

 weathered rock that it is necessary to remove in order to reach struc- 

 turally sound material is usually slight. In the case of the University 

 dam again, the maximum depth of excavation into rock was about five 

 feet. On the crests of narrow ridges the rock will be found to l>e 

 weathered to a greater depth than the above figure. lUit under the allu- 

 \ium of valleys, and on the sides of steep hills, the depth of weathered 

 :tnd fractured rock should sebl<)m be great'. 



The Riverside sand.^tone constitutes ai>i)roximately the ui>per 100 feet 

 of the Knobstone formation. I'.elow this are alternating shales and 

 sandstones, with the shale ]>redominating. This shale is sandy or ai'gil- 

 laceous, and toward llie lower i)art of the formation, as may be seen 

 in the ([uarrles of th(» l.ohigh Cement Company, at I'.rownstown, it 

 becomes dark colorwl and somewhat carbonaceoiis. When unweathered the 

 shale is firm and tough, and shows, on account of its sandy character, 

 very little tendency to slip under heavy loading. In the excavation of 

 the cuts on tlie Illinois Central, most of th(> sli;ile reipnred heavy 

 blasting, and like the sandstone, described abov(>. was tough and hard 



* The reason for this is clrar ciiohkIi, whon it is rL>mpnihorcd that the onl.v 

 agents of weathering tliat mafcrially aflfect this i-oek are meelianical, such as 

 insolation, frost action, and Hio wedging action of ireo roots; and that unlike the 

 liiiicstone, presently to lie discussed, it is not at all affectc-d by solution — an agent 

 I hat fi<"ts to rpueh greater depths, 



