Supraorbital ridge

“ Subsequent erosion has obliterated in great part the inequality of the surface, and the higher-lying blocks do not stand as mountains, but are outlined by zones of tilted strata which mark the flexures by which the blocks are separated. Along the zones of flexure it frequently happens that a hard stratum outcropping between two that are softer will be preserved from erosion and form a long, continuous ridge. Such ridges, and other forms produced by the erosion of the flexures, are conspicuous features of the topography, and the tracing out of the limits of the blocks is a simple matter. ”

“ In the intervening low district a basaltic andesite is exposed in the stream-courses. The structure of the ridge, as indicated by the ascent of its western slope, is shown in the accompanying diagram. The core or central axis is formed of massive basic rocks which protrude at the summit and in one or two of the crests of the spurs. The flanks are composed of submarine tuffs and clays overlaid by agglomerates of considerable thickness. The tuffs reach to within 50 feet of the top, whilst the agglomerates extend to within 400 feet of the summit. ”

“ Sometimes the hardest stratified rock-beds resist the water and the wind and are left as a series of ridges along the sides of the main range. The crumpling forces may crack the ridge open for its whole length, and one side of the chasm may slip down and the other go up. The result is a sheer wall of exposed rock strata, layers of which correspond with those that lie far below the top of the portion that slid down in the great upheaval and subsidence that parted them. ”

“ The crest of this first main ridge and the surrounding mountains, with the exception of a few lofty pinnacles, are capped by a great thickness of a horizontally stratified, tufaceous deposit. The lowest bed is of a pale purple colour, hard, fine-grained, and full of broken crystals of feldspar and scales of mica. The middle bed is coarser, and less hard, and hence weathers into very sharp pinnacles; it includes very small fragments of granite, and innumerable ones of all sizes of grey vesicular trachyte, some of which were distinctly rounded. ”

“ They are better observed on the northern than on the opposite bank, being on the former very much exposed by the operations of the quarrier.
Upon the top of this ridge and upon the fractured edges of the limestone strata, behind Clifton, there has been a partial deposition of stratified yellow sandstone, forming what has been called an upfilling. In the fissures of this rock crystals of carbonate of lime are found, and crystals of sulphate of strontian, which often assume a radiated form. This sandstone, having sometimes the appearance of a breccia extends to Redland. ”

“ The greater part of the hill is formed of a coarse tuff, whose fragments stand out in sharp relief on the surface of the huge projecting, bowlder-like masses. The matrix, which is darker and softer, weathers out, leaving the surface of the rock covered with a great variety of fragments. Some are scoriaceous or amygdaloidal, others are very coarsely crystalline and porphyritic with feldspar or hornblende. Some of the fragments are themselves tufaceous, containing large masses of hornblende in a dioritic matrix. On the southern end of the hill, a great variety of dikes intersect the tufa. ”

“ It often happens that in the interval between the deposition of two sets of unconformable strata, the inferior rock has not only been denuded, but drilled by perforating shells. Thus, for example, at Autreppe and Gusigny, near Mons, beds of an ancient (primary or palæozoic) limestone, highly inclined, and often bent, are covered with horizontal strata of greenish and whitish marls of the Cretaceous formation. The lowest, and therefore the oldest, bed of the horizontal series is usually the sand and conglomerate, a, in which are rounded fragments of stone, from an inch to two feet in diameter. ”

“ The ancient granite ridge of the Laurentian Hills is now low, and slopes gently. This is true of all very old mountains. The newer ones are high and steep. It takes time to grind down the peaks and carry off the waste material loosened by erosion.
Far more material than could have been washed down the slopes of the first land ridges came directly from the interior of the earth, and spread out in vast, submarine layers upon the early crust. Volcanic craters opened under water, and poured out liquid mineral matter, that flowed over the sea bottom before it cooled. ”

“ This ridge consists of secondary limestone, overlying a coarse reddish sandstone, which, at the lowest stage of the water in summer, is seen in huge misshapen fragments, at the immediate edge of the water, and at intervals nearly half way across the river, as well as on the Illinois shore. The mineral occurs in mass, abundantly. It is nearly dry, of a perfectly white color, and chalky friability. It embraces masses of hornstone, resembling flint. It also occurs at a higher point on the same shore, two miles below the Grand Tower. ”

“ Now the alterations above described as superinduced in rocks by volcanic dikes and granite veins, prove incontestably that powers exist in nature capable of transforming fossiliferous into crystalline strata—powers capable of generating in them a new mineral character, similar, nay, often absolutely identical, with that of gneiss, mica-schist, and other stratified members of the hypogene series. The precise nature of these altering causes, which may provisionally be termed plutonic, is in a great degree obscure and doubtful ”

“ If, however, we pass on to the east side of the Lickey quartz ridge, we shall find the natural boundary of the red marls coming up within a mile of the older rocks, and near Kendal end, within 100 or 200 yards of them. As the beds do not seem to have any greater amount of dip on the east of the quartz ridge than they have on the south-west, we must either suppose the whole of the red sandstone to have thinned out here, and the red marls to overlap nearly the whole of the lower beds, or a very considerable downthrow fault to traverse the east side of the quartz ridge. ”

“ Angles are never formed by a single ridge but by the abutting of one ridge against another. Therefore, an angular formation cannot be used as a type line. In figure 18, ridges A and B join at an angle. Ridge B does not run parallel with ridge D; ridge A does not diverge. Ridges C and D, therefore, are the type lines.
[Fig. 18]
Focal points—Within the pattern areas of loops and whorls are enclosed the focal points which are used to classify them. These points are called delta and core. ”

“ But the theory adopted in this work of the subterranean origin of the hypogene formations would be untenable, if the supposed fact here alluded to, of the appearance of tertiary granite at the surface was not a rare exception to the general rule. A considerable lapse of time must intervene between the formation in the nether regions of plutonic and metamorphic rocks, and their emergence at the surface. For a long series of subterranean movements must occur before such rocks can be uplifted into the atmosphere or the ocean ”

“ To the south of this ridge, open sea extended far southward and westward over the site of the Mendip Hills and the uplands of South Wales.
The Carboniferous period, as chronicled by its sedimentary deposits, was a time of slow submergence and quiet sedimentation, terrestrial and marine - 356 - conditions alternating along the margins of the sinking land, according as the rate of depression surpassed or fell short of that of the deposition of sediment. There is no trace of any general disturbance among the strata, such as would be marked by an important and widely extended unconformability. ”

“ After ascending for half an hour the western ridge bounding the Kosloo valley, where a mass of limestone decidedly overlies the coal-bearing deposits, we reached and followed the backbone of a ridge nearly 2000 feet above the sea, where the shales and schists composing it appeared to be but little inclined from the horizontal position, but whether conformable or not with the former or latter could not be determined ”

“ At the commencement of the ridge (2,100 feet) is displayed an altered hypersthene-augite andesite, rudely columnar blocks of which, up to 2 feet in diameter, were lying about. It belongs to genus 1 of this sub-class (see page 286) which also includes the rocks exposed farther along the ridge. In these rocks the felspar-lathes are small (·05-·07 mm. long) and are not in flow arrangement. The interstitial glass varies in amount, and the specific gravity is about 2·7.
The ascent is very gradual for the first one and a half miles, when an elevation of 2,600 feet is attained. ”

“ Indeed it is not clear that the fullest resources of stress-accumulation which the spheroidal form of the earth affords are sufficient to meet the demands of the problem, unless the rigidity of the earth be taken at a much higher value than that of surface-rock, and this is perhaps an additional argument for the high rigidities inferred from tides and seismic waves.
In view of the doubtful competency of even the thickest segments to accumulate the requisite stresses, there is need to consider modes of differential stress-accumulation other than those dependent on sphericity. ”

“ To merely drive down iron rods feeling for so-called bed rock, or to make only a few bore-holes with an earth auger should in no instance be considered sufficient. Borings may be found necessary at considerable depths below the surface and in certain classes of material, but dug pits or shafts should always be resorted to for moderate depths and whenever practicable. Only by such means may the true character of the strata underlying the surface, and the nature and condition of the bed rock, if it be reached, become known. ”

“ Much of the upper and more rapid circulation is lateral, being actuated by the sloping surface of the ground-water, which in turn is determined by topography, precipitation, and other surface conditions. Theoretical considerations, therefore, favor the view that lateral flow is an important factor in the concentration of ore material. But as descending and lateral currents almost inevitably meet and mingle with ascending currents, it is difficult to distinguish, in the ore-deposits, the special functions of each phase of action. ”

“ For example, if the hypothetical level of no stress is eight miles below the surface, as computed on certain assumptions, the concave portion must be so broad that the isogeotherms will also be concave outward at something near that depth; in other words, the main part of the zone of thrust must be concave. A narrow concavity at the surface, such as an ordinary valley in a portion of the crust that has the average convexity, would not seriously depress the isogeotherms, or affect the zone of thrust, but a valley several times eight miles (level of no stress) in breadth would. ”

“ Nor must it be supposed that the whole scale of sedimentary formations is to be found superimposed in one spot; for as in our times new strata are chiefly growing at the mouths of rivers, or where submarine currents deposit at the bottom of the ocean the fine mud or sand which is conveyed into the sea by the disintegration of distant mountain chains, so also from the beginning each stratum could only have been deposited in similar localities ”

“ Regional metamorphism may subsequently induce new structures or minimize the effects of the old. Joint systems may be developed, the planes widely spaced in one group of rocks giving rise to monolithic masses very resistant to the agents of weathering, while those of an adjacent group may be so closely spaced as greatly to hasten the rate of denudation. There may be developed so great a degree of schistosity in one rock as to give rise (with vigorous erosion) to a serrate topography; on the other hand the forms developed on the rocks of a batholith may be massive and coarse-textured. ”

“ If we admit what is perhaps impossible to be denied that the production of each sort of rock spread from some centre, and that these centres were not coincident for different rocks, it becomes a very curious problem to determine what are the lines of contemporaneity in the oolitic system.
For let A be a point from whence a deposition of carbonate of lime spreads slowly through the ocean, but not reaching to B, a point from which depositions of sand happen, not reaching to A—the general basis r, r, r, being red marl and sandstone. ”

“ When the associated gangue and country rock are less soluble than the copper—as is common with quartz and igneous rocks—the oxide zone is likely to be depleted of values.
The zones formed by weathering and secondary enrichment are extremely irregular, both in distribution and depth, in any one deposit, and they overlap and grade into one another in a very complex fashion. In many places the primary zone is too lean to be mined to commercial advantage; but in other places, as at Butte, and in the limestone deposits of Bingham, the primary ores are of considerable importance. ”

“ The lower limit of the ground-water is more irregular than the upper surface and is less definitely known. In general, openings in rocks tend to diminish with depth, due to cementation and to closing of cavities by pressures which are too great for the rock to withstand. But rocks differ so widely in their original character, and in their response to physical and chemical environment, that it is not unusual to find dense and impervious rocks above, and open and porous rocks below. The lower limit of the zone of abundant underground water varies accordingly. ”