The most recent of the formations which are pierced by the drill, but which do not come to the surface within the limits of the county, is the _Maquoketa shale_, reached in the eastern townships at a depth of somewhat more than 300 feet. This impervious bed of altered clay stops the descent of ground water, which thus is stored in large quantities in the overlying limestones and supplies some of the important wells of the county such as that of the town of Mount Vernon. At the time when these sea clays were laid, eastern Iowa was under sea, but so near was the low lying land to the north and east that vast quantities of mud were brought in by its rivers forming deposits nearly 300 feet in thickness. THE SILURIAN With the lapse of ages physical conditions changed and Linn County was covered with a warm shallow coral sea in which were laid the massive limestones which now form the country rock in the eastern tier of townships. In some of the quarries one may see the ripple marks into which these coral sands were heaped by the pulse of the waves, and one may pick out of the rocks casts and moulds of ancient sea shells, corals, and trilobites, which formed the highest forms of life then tenanting the Iowa seas. The lowest beds of the Silurian belong to the _Hopkinton stage_, and are exposed along the Buffalo. At Hill's mill and at Nugent's quarries some layers are crowded with a characteristic fossil--a plump bivalve shell as large as a walnut, which goes by the name of _Pentamerus Oblongus_. The _Gower_ stage of the Silurian rests upon the Hopkinton and embraces two types of rocks distinct in their appearance and uses. The _LeClaire phase_ of the Gower is a hard, brittle, crystalline, magnesian limestone, or dolomite. Normally blue-gray in color, it is often oxidised to buff. It is well exposed at Viola and on the Cedar river from the Cedar County line to a mile or so beyond the Upper Palisades, southwest of Bertram. The LeClaire forms mounds in places reaching fifty and even eighty or ninety feet in height in which little semblance of bedding structures are to be seen. Here and there the rock is conglomeratic, consisting of rounded masses of the rock cemented by a less resistent matrix. The cavernous recess in the rock wall of the Palisades, misnamed the Blowout, is due to the solution of the weaker matrix and the dislodgement of the rounded masses. The rock may consist also of angular broken blue-gray fragments in the matrix of a buff and friable limestone sand. Again, the mounds, at least in part, may be made up of massive limestone with little trace of structure of any sort. On the sides of the mounds and merging into the conglomeratic or other structures the rock of the LeClaire often is stratified and the layers dip outward at angles surprisingly high. In places these tilted layers may show sharp folds. The rock of all structures is fossiliferous. Even the broken fragments of breccia are porous with