The rocks here concerned are the extension of the same stratified Palaeozoic formations already described as occurring~in the Appalachian region and around the Great Lakes.
Ignoring temperature effect, and taking the density as a function of the pressure, surfaces of equal pressure are also of equal density, and the fluid is stratified by surfaces orthogonal to the lines of force; n ap, dy, P d z, or X, Y, Z (4) are the partial differential coefficients of some function P, =fdplp, of x, y, z; so that X, Y, Z must be the partial differential coefficients of a potential -V, such that the force in any direction is the downward gradient of V; and then dP dV (5) ax + Tr=0, or P+V =constant, in which P may be called the hydrostatic head and V the head of potential.
The lower Nile valley, however, forms an exception; flint implements of a palaeolithic type have been found near Thebes, not only on the surface of the ground, which for several thousand years has been desert owing to the contraction of the river-bed, but also in stratified gravel of an older date.
The oldest rocks in this large area are a stratified series of mica-schists, limestones and quartzites, with numerous intrusive sheets of diorite, the whole having been metamorphosed by pressure, with frequent overfolding.
There are large deposits of stratified clay along the shores of Lake Michigan, from which is made a cream-coloured brick, so largely used in Milwaukee that that city has been called the "cream city"; the total value of clay products in 1907 was $1,127,819 and in 1908 $95 8, 395.