OO4. GEOLOGY OF OHIO. 
twelve feet below Coal No. 6—and the representative of the drab lime- 
stone of Columbiana county, often found directly beneath No 6. In Co- 
shocton county this “Black Marble” often passes into a chert, as do all 
the limestones of that county, but none of them form so extensive and 
continuous deposits as the flint of Flint Ridge. Any one traversing 
this ridge for the first time would be surprised to find such a deposit on 
such a geological horizon. It simulates wery accurately the broken-up 
debris of a vertical dike, the fragments often covered with perfect 
erystals of quartz, the rock itself being highly crystalline and often 
translucent. It is something of a puzzle to understand how such a de- 
posit is found in a series of undisturbed and unmodified sedimentary 
rocks. The adjacent surfaces of two blocks of the chert are often found 
covered with quartz crystals of considerable size, as thoroughly inter- 
locking with each other as if one were a cast, and the other the matrix. 
I can not imagine conditions which would spread such a deposit over the 
floor of a sea or any other body of water. <A substitution of silicious 
matter deposited from solution, in the place of a soluble limestone pre- 
viously deposited, is the only plausible explanation. This substitution 
has taken place over large areas in this part of the State, and has left 
these silicious deposits only upon the horizons of the different lime- 
stones.* 
The coal immediately below the flint is indicated by outcrops in var- 
ious places, but wherever observed is thin, and apparently of no value. 
Below is a bed of fire-clay two to four feet in thickness, which appears 
to be of good quality. 
A few hills rise to the height of eighty feet, by the barometer, above 
the flint, showing debris of chert and sand-rocks. At an elevation of 
seventy feet above the flint in one place, a heavy outcrop of fire-clay was 
* The question of the origin of the silica, which so often replaces the carbonate of lime 
in the Coal Measure limestones, is discussed at some length in Vol. If. of this report, and 
it is there attributed to Diatoms. These microscopic plants, as is well known, bear sili- 
cious frustules, which accumulate at the bottom of some lakes and ponds till they form 
beds many miles in extent and several feet in thickness. They probably inhabited por- 
tions of the shallow land-locked basins where the limestones were formed in such num- 
bers as to supply silica for concretions or cherty layers, and sometimes to replace the 
calcareous bed entirely, just as we find the diatomaceous earths locally replacing shell- 
marl in the bottoms of our lakes and marshes. The silica which forms the frustules of 
the diatoms has been proved by experiment to be unusually soluble, and in the flint 
beds, the individual forms have doubtless been either so completely dissolved or so envel- 
oped in soluble silica as to be lost. The quartz crystals referred to by Mr. Read as coat- 
ing the blocks and filling the crevices and cavities of the flint, are evidently of modern 
origin, and have been formed by a deposit of silica, from solution, in whatever recepta- 
cles were open toit. (See Vol. Il, Part I., page 142.) 
