Thus comparing the Nearctic and Palaearctic floras we find striking differences overlying the points of agreement already indicated.
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.
That it results from the weight of the overlying strata.
In thickness, occupying undulating hollows in the underlying gneiss, and dip towards the Noursoak Peninsula at 20°, when the overlying Atanakerdluk strata come in.
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.
The overlying sandstones afford good building stones, and frequently, as at Vereeniging, yield many fossil plants.
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.
Sacrifice a considerable proportion of the mineral, which is left for the support of the overlying strata.
In the systematic mining of larger deposits, the simplest plan consists in mining large areas by means of numerous working-places under the protection of pillars of mineral left for the purpose, and later mining these pillars systematically, allowing the overlying rock beds to fall and fill the abandoned workings.
Rock-filling yields and becomes consolidated under heavy pressure, and therefore does not furnish a rigid support of the overlying strata, but rather a cushion to control and equalize the subsidence.
Above, the ground has been completely worked out from the surface, and the space formerly occupied by ore is now filled with the debris of the overlying strata which has caved in above the block of ore now being worked.
On completion of any room the timbers are withdrawn and the overlying mass of timber and rock is allowed to fall and a new room is started immediately alongside of the one just completed.
In this way the whole floor is worked out and the mat of timber and overlying rock is gradually lowered and rests upon the top of the ore forming the floor below.
'Where mineral deposits lie near the surface underground mining may be replaced by open excavations, and the reduced cost of mining makes it possible to remove the overlying soil and rock to considerable depths.
The special difficulties which attend deep mining, in addition to the problems of hoisting ore and raising water from great depths, are the increase of temperature of the rocks and the pressure of the overlying strata.
While the mine workings are small the overlying rocks support themselves of and the full pressure does not come upon the mine Caving p i llars.
Its upheaval above the great sea which submerged all the north-west of the Indian peninsula long after the Himalaya had massed itself as a formidable mountain chain, belongs to a comparatively recent geologic period, and the same thrust upwards of vast masses of cretaceous limestone has disturbed the overlying recent beds of shale and clays with very similar results to those which have left so marked an impress on the Baluch frontier.
The Cretaceous beds have not yet been separated from the overlying Eocene, and the identification of the system rests on the discovery of a single Cenomanian ammonite.
The soils overlying red sandstone rocks would be reddish and of a sandy nature, while those overlying chalk would be whitish and contain considerable amounts of lime.
The deposits in the great limestone caves of Kentucky, Virginia and Indiana have been probably derived from the overlying soil and accumulated by percolating water; they are of no commercial value.
Overlying these amongst the Palaeozoic rocks, we meet in many parts of Japan with slates and other rocks possibly of Cambrian or Silurian age.
In many cases we find large portions of the surface completely covered with green or yellow enamel overlying black diapers or scroll patterns.
Its diaphanous, pearl-grey glaze, uniform, lustrous and finely crackled, overlying encaustic decoration in white slip, the fineness of its warm reddish pate, and the general excellence of its technique, have always commanded admiration.
Between this lowland and Armagh city, the early Cainozoic basalts form slightly higher ground, while on the west a strip of Trias appears, overlying Carboniferous Limestone.
In alluvial deposits the richest ground is usually found in contact with the "bed rock"; and, when the overlying cover of gravel is very thick, or, as sometimes happens, when the older gravel is covered with a flow of basalt, regular mining by shafts and levels, as in what are known as tunnel-claims, may be required to reach the auriferous ground.
In the overlying air.
The temperature of the surface water is generally a little higher than that of the overlying air, the daily average difference being about o 6° F., varying from 0.9° lower at I P.M.
3; the succeeding stages of creep are shown at b, c, d, f, and g, in the same figure; the last being the final stage, when the coal begins to sustain the pressure from the overlying strata, in common with the disturbed pavement.
Rift valley, overlying a formation of granite, gneiss and quartz.
The Cambrian rocks of Ireland, a great series of purple and green shales, slates and grits with beds of quartzite, have not yet yielded sufficient fossil evidence to permit of a correlation with the Welsh rocks, and possibly some parts of the series may be transferred in the future to the overlying Ordovician.
Through these, again, pierce other granites in dikes or lava flows, and overlying the whole are limestones of Cretaceous and Tertiary age, themselves cut through by later volcanic eruptions.
When the land reappeared a new series of valleys would at once begin to be eroded; and the subsequent degradation of these overlying sediments might reveal portions of the older topography, as in the case of the Great Glen, Lauderdale, and other ancient valleys.
Determined by the inequalities of surface of the overlying mantle of sedimentary material, they would be wholly independent of the geological structure of the rocks lying.
Slowly sinking deeper and deeper into the land, they might eventually reach the older rocks, but they would keep in these the lines of valley that they had followed in the overlying deposits.
The most impressive form of solitary cone is that wherein after vast denudation a thick overlying formation has been reduced to a single outlier, such as Morven in Caithness, the two Bens Griam in Sutherland, and still more strikingly, the pyramids of red sandstone on the western margin of the shires of Sutherland and Ross and Cromarty.
The rocks overlying them to the east of the line of disturbance in the shires of Sutherland and Ross and Cromarty are fine flaggy schists.
Rocks belonging to the Cretaceous system at one time covered considerable areas on both sides of the Highlands, but they have been entirely stripped off the eastern side, while on the western they have been reduced to a few fragmentary patches, which have survived because of the overlying sheets of basalt that have protected them.
They are prolonged southwards into Antrim, where similar basalts overlying Secondary strata cover a large territory.
To the south-east of this region a narrow outcrop of Upper Llandovery, Wenlock and Ludlow sandstones and mudstones follows, uncomformably overlying the Llandeilo and Bala rocks, and dipping conformably under the Old Red Sandstone; they extend from Newbridge-on-Wye and Builth through Llangammarch (where there are mineral springs) towards Llandovery, while a tongue of Ludlow rocks brought up by faulting extends from Erwood on the Wye for 8 m.
In Cheshire the surface-water trickling through the overlying strata dissolves the salt, which is subsequently pumped as brine, but at Middlesbrough the great depth and impermeability of the strata precludes this, so another method has been resorted to.