TUSCAEAWAS COUNTY. 63 



toms), which secrete pure silica to form their frustules. These micro- 

 scopic organisms inhabit nearly all lakes and ponds, and in some 

 instances produce strata many feet in thickness of silicious earth (" tri- 

 poli," " infusorial earth," etc.)- This is often found associated or alterna- 

 ting with shell-marl, precisely as the limestone and flint are mingled, 

 the only marked difference between the marl and limestone, and the tri- 

 poli and flint, being in their degree of solidification. As the silica 

 secreted by diatoms is very soluble in alkalies, we may infer that the 

 older deposits of this kind have become infiltrated and solidified until 

 they are converted into chert of flint, just as the soft marls have been 

 hardened into limestones. 



In certain localities both the Putnam Hill and Zoar limestones contain 

 so much earthy matter as to be unfit for burning into quicklime. This 

 is specially true of the Putnam Hill limestone, and one outcrop of this 

 character, at New Philadelphia, has been already mentioned. Where 

 this phase is assumed, and the quantity of silica and alumina is not too 

 great, a hydraulic limestone or cement rock is found. No specimens of 

 either limestone were found in Tuscarawas county which seem to prom- 

 ise to produce a cement of first quality, but not the hundredth or thous- 

 andth part of the outcrops of the strata could be examined, and since the 

 limestones are very variable, there may be many deposits of the requisite 

 character which are as yet unknown. It would be well, therefore, for 

 the inhabitants of the county to have their attention dr.iwn to the possi- 

 bility of finding a good cement rock in some local phase of one or the 

 other of the two limestones which underlie every township. The best 

 cement rocks are usually fine-grained, with a smooth conchoidal fracture. 

 In burning they do not swell much, and generate little or no heat on the 

 application of water after calcination. When ground to powder, and 

 made into a paste, however, they have the peculiar property of harden- 

 ing under water. 



Chemical analyses will generally not determine the value of the mate- 

 rial supposed to possess hydraulic properties. The best test is actual 

 experiment, which may be performed by any intelligent person. Small 

 fragments of the stone to be tried should be calcined in a common cruci- 

 ble, samples being taken out and tested from time to time, until the 

 pieces left in the crucible begin to vitrefy. When burnt to diflerent de- 

 grees, the fragments should be separately pulverized in an iron mortar, 

 wet up with water to the consistence of putty, and laid in water to 

 harden or dissolve, according to their properties. 



Iron Ore. — As has been mentioned in the general description of the 

 Putnam Hill and Zoar limestones in Vol. II, the surface of each of these 



