122 



Central Kailway, and aj^ain in the excavation t'nv the rmnHliition of the 

 Indiana University dam. When, however, the rock is exposed to the 

 notion of the sun in sumiuer, and of frost in winter, the differential expan- 

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

 of interstitial uatcr in the otlier, rapidly reduce the rock to a mass of 

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

 the sandstone is of no at-couiit as a building stone. The peculiarities of the 

 rock, just enumerated, are duo in large measure to the tineness of the 

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

 moist, gives the roclv its unique toxigliness and impermeability. 



Structurally also this sandstone is extremely f'avoralile as a sub- 

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

 planes. In the case of the University dam, which is llGi feet long at the 

 base and 34 feet high above the roclc, there is not a single joint or 

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

 the dam are in perfectly sound and unfissured 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 dei)th of excavation into rock was about five 

 feet. On the crests of narrow ridges the rock will be found to \ye 

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

 vium of valleys, and on the sides of steep hills, the depth of weathered 

 and fractured rock should seldom be great'. 



The Riverside sandstone constitutes approximately the upper 100 feet 

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

 sandstones, with the shale predominating. This shale is sandy or argil- 

 laceous, and toward the lower part of the formation, as may be seen 

 in the quarries of the Lebiu'li Cement C<Hnitany. at I'rownstown. it 

 becomes dark cojoi'ed and suniewlial ( arbonaceons. When uiiweatliered the 

 shale is firm and fough, and shows, on ;icconnf of its sandy character. 

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

 the cuts on tin Illinois Central, most of the slinle re(|uired heavy 

 blasting, and like the sandstone, di'scribed above, was tongh and hard 



1 The reason for this Is clear onoiifjli, wlicii it is r Miit'iulM'n'd that the only 

 agents of weathering that materially alToet this mck ;ir.' mechanical, such :is 

 insolation, frost action, and tlie wodpring action of ucc rndis: :md that nnlllie tin 

 limestone, pi-es"ntly to lie discussed, it is not at all alTfcli-d by scdntion— an a.i;ei>t 

 Ihjjf arfs to niMch ^'|'e:i|er depths, 



