As a dolomite the stone is far more resistant than a purer limestone. In the Mount Vernon cemetery tombstones of this material, whose dates run back to the forties and early fifties, have been so little affected by superficial decay that the tool marks are almost as fresh as when the chisel left them; while marbles of half their age have broken down into ruin. The Silurian rocks of the county measure about 300 feet in thickness. They are confined pretty closely to the townships of the eastern tier, but extend beyond their limits up the valleys of the Cedar and Wapsipinicon. THE BERTRAM LIMESTONE As the Silurian limestones sink below the surface because of the westward dip, they are succeeded by a bed of rock, named from its outcrop at Bertram, and found along Big Creek as far north as Paralta and Springville. This is a heavily bedded gray rock which weathers almost white. At a number of places along Big Creek it forms picturesque cliffs, and hillsides covered with huge boulders of disintegration. At one point it is seen to overlie the Anamosa beds of the Silurian, and several exposures are known where it is succeeded by the Otis limestones of the Devonian. But as it contains no fossils, so far as is now known, it can not be said to which of the two ages it belongs. THE OTIS LIMESTONES The lower beds of the Otis, as exposed at the base of the Otis quarries, along the Cedar south of Cedar Rapids, at Springville, and at Coggon, consist of soft magnesian limestones, fossiliferous with many moulds of small bivalve shells of Devonian age. These pass upward into drab non-magnesian limestones carrying the principal fossil of the magnesian beds in considerable numbers. The upper limestones of the Otis differ within rather wide limits. The most common type is seen at the base of the high cliff at Kenwood on the right bank of Indian Creek--a hard, brittle ringing and thinly laminated limestone. Often it has been subjected to strains under which it has broken, and has been re-cemented with little displacement of the parts. Occasionally it is brown, and highly crystalline. THE INDEPENDENCE At the Kenwood cliff the eight feet of the Otis at the base is succeeded by thirty feet of buff shale and clayey limestones--a formation known as the _Independence_ from its discovery in a shaft sunk at that city. The Independence is exposed at many points near Cedar Rapids both on Indian Creek and on the Cedar. On the Wapsipinicon it is well seen at Cedar Bluff (sec. 24 Spring Grove Tp.), at the "Wolf's Den" a mile up valley, and again in the railway cut north of Coggon. In the long cut of the Chicago, Milwaukee & St. Paul Railway west of Linn Junction the Independence is seen in one place as a blue clay shale carrying a number of fossils characteristic of the shaft at Independence, but elsewhere the formation is unfossiliferous in the natural exposures so far studied.