PLATE 4 
A. —Concrete tile laid in Central Plain of Wisconsin in 1915, taken up in 1919. No. 
I lay on the ground. No. 2 was in sand below 8 inches of black muck. No. 3 was in 
silt loam with sand at 3 feet 6 inches. No. 4 was in bowlder clay underlying 12 inches 
of black muck. No. 5 was in sand below 8 inches of black muck. 
B. —Concrete tile laid in a sewer in Milwaukee about 1885. Taken up in 1919. 
