That it results from the weight of the overlying strata.
Thus comparing the Nearctic and Palaearctic floras we find striking differences overlying the points of agreement already indicated.
A considerable part of it has been excavated by these rivers to a level which gives their valleys the elevation and character of lowlands, though isolated hills and ranges with the characteristic overlying horizontal sandstone strata of the ancient plateau show that it was once a highland region.
Drifts and inclined shafts following the deposit may prove difficult of maintenance when the workings become large and settlement of the overlying strata begins.
While it is always desirable to provide large working-places, the size of the working-place is limited by the thickness and Size of strength of the overlying beds forming the roof Working- or hanging wall of the mine.