STATE GEOLOGIST. 39 
cement. If the surface is left in the wet condition and not protected from 
air and sun, it will dry out very quickly and in common with other ma- 
terials in a moist state when drying in this manner it will crack.” 
Figure 3 shows the initial appearance of hair-cracks as illustrated upon 
the parapets and corner posts of the Y bridge at Zanesville. Figure 4 
Fig. 4.—Concrete Steps, Showing Developed Hair Cracks. 
shows the approaching dissolution of some concrete steps affected with 
hair-cracks. These steps have been down twenty-two months. 
As the fine sand and over-troweling both develop an excess of cement 
at the surface, there are really 16 answers favoring over-rich mortar as the 
cause, with 6 answers scattered between troweling after initial set, rapid 
drying, and the after hydration of excess free lime. 
It has been the writer’s opinion based upon several years of observa- 
tion both of the construction and the wear of cement walks, that such 
cracks are caused by the expansion and contraction of the immediate 
