Thus comparing the Nearctic and Palaearctic floras we find striking differences overlying the points of agreement already indicated.
That it results from the weight of the overlying strata.
Overlying the Tuscaloosa are the Eutaw sands, characterized by sandy laminated clays, and yellow, orange, red and blue sands, containing lignite and fossil resin.
Similarly it is necessary, in view of the hydrostatical relations of water and mineral oils, and the volatile character of the latter, that the porous stratum should be protected from water and air by an overlying shale or other impervious deposit.
In thickness, occupying undulating hollows in the underlying gneiss, and dip towards the Noursoak Peninsula at 20°, when the overlying Atanakerdluk strata come in.