Dolomite

“ Many of the insulated rocks are perforated. Between the bold projecting cliffs of limestone there are narrow shelving beaches, formed of its debris, that afford access to the interior. The strata have generally a slight dip to the northward, and the most common Rock is a yellowish-gray dolomite which has a very compact structure, but presents some shining facets of disseminated calc-spar. ”

“ Hence, it is entirely an arbitrary matter as to where we shall draw the line between dolomitic limestone and calcareous dolomite. Dolomite is a very much less abundant rock than limestone, and, unlike limestone, it rarely contains many fossils, and is never of organic origin; i.e., there are no organisms which secrete the mineral dolomite to form their hard parts or skeletons. Like gypsum and rock-salt, dolomite is probably never deposited in the open ocean, but only in closed basins. Like limestone, it occurs with both the compact and the crystalline textures. ”

“ Dolomite.—If for calcite, which is the sole essential constituent of all limestone, we substitute the allied mineral dolomite, we have the rock dolomite. As might be inferred from its composition, dolomite is very closely related to limestone, although there are some important differences. Physically, the two rocks differ about as the two minerals do. Dolomite is harder than limestone, and being also less soluble, it resists the action of the weather more. Dolomite, if pure, effervesces feebly, or not at all, with cold dilute acid. ”

“ It is very interesting to note that dolomites are very frequent among rocks which indicate that desert or salt-lake conditions prevailed at the time of their deposit. The dolomite or magnesian limestone of the English Permian is an instance of this. The explanation may be found in the fact that the waters of bitter lakes are usually rich in magnesium salts which, percolating through beds of limestone, would convert them into dolomite. Among the most famous dolomites are those of the Dolomite Alps of Tirol. They are of Triassic age and yield remarkably picturesque mountain scenery ”

“ Just about the time this surge of water reached the mouth of the canyon, half of a 7,600-ft.-high mountain came crashing down into the valley and cascaded, like water, up the opposite canyon wall, hurtling house-size quartzite and dolomite boulders onto the lower portion of Rock Creek Campground. ”

“ Most of them, indeed, will admit that they cannot afford to neglect the Dolomites, and will at least intend to spend a season amongst them. From a scrambling point of view, if they are rock-climbers, they will be well rewarded, for Dolomite [85] rock-climbing is a thing apart. Dolomite scenery is even more truly unique. Less grand than that of the great mountain groups, it has a distinction all its own. ”

“ These properties assume more significance in mineral determination as one becomes familiar with mineral collections, but the non-technical person can rely on the acid test.
Typical dolomite crystals from Joplin region.
With the above information in mind, one may proceed with certainty to identify a layer of dolomite from a quarry or hillside, or a crystal of it in a hand specimen. First, determine that it is scratched readily with a knife blade or iron nail. Anything too hard to be scratched by steel is neither calcite nor dolomite. ”

“ The landscape, which has from the first possessed a peculiar charm for us, is often blotted from view. The varied, undivided yet most individual range of dolomites which rises on the edge of the eastern horizon, instead of melting under the soft influences of warm sunshine and quivering air into glowing crimson, purple, palpitating mountains—which only with advancing night turn into gray, motionless pinnacles and battlements of the great dolomitic land that stretches beyond—now remain, whenever visible, cold, hard masses of snow, like rigid nuns of some ascetic order. ”

“ The highest limestone rocks in this region, those which rise from the plateau or causse and strike the imagination by the strangeness of their forms, are dolomite; in the gorges they approach the character of lias towards the base, and not unfrequently contain lumps of pure silex embedded in their mass. The redness which they so often show, and which, alternating with yellow, white, or gray, adds to the grandeur of their rugged outlines, is due to the iron which the rock contains. ”

“ A large region in southern Minnesota, southern Wisconsin, and northern Iowa has never been glaciated and is known as the driftless area. In this region the great river and its tributaries have cut deep valleys through layers of limestone, dolomite, and sandstone. The sides of the valleys have never been rounded off by creeping glaciers, and the cliffs of dolomite stand up straight and bold like the well-known Maiden Rock and Sugar Loaf near Winona. ”

“ The dolomites of the Jurassic series in north Bavaria are massive rocks almost devoid of fossils, traversed by shrinkage cracks, and associated with richly fossiliferous stratified limestones. The relations of these two types of rock are those of coral-reefs to the bedded deposits on their flanks, and the dolomite seems to merge horizontally into the stratified series. As in Tyrol, fossils and corals are rare in the bosses of dolomite, but the structural evidence is strongly in favour of their having originated as steeply sided reefs. ”

“ The intimate structure of modern dolomitic limestone, as exhibited in coral-reefs, satisfies us that many older or fossil dolomites were formed from marine calcareous deposits while these were still accumulating. In other cases we must admit that the dolomite has developed in the neighbourhood of joints after the consolidation of the rock. The view that dolomitisation results from the mere removal of calcium, the magnesium originally present in organic skeletons becoming thus more concentrated, is not borne out by recent observations. ”

“ The heavy bed of tough dolomite limestone at the crest of the falls, which is the occasion of their existence, lies above a thick mass of soft shale which easily caves in under the rebound of the falling waters, and by so doing becomes the chief cause of the breaking down of the crest and of the cataract's retreat. ”

“ Calcite is more soluble than dolomite in water saturated with carbonic acid and would tend to be slowly removed from a limestone, while the dolomite increased in relative proportion. Dolomite also being denser than calcite may be supposed to replace it more readily when pressure is increased. These and many other factors probably co-operate to effect the transmutation of limestones into dolomites.
Examples of dolomitization may be obtained in practically every geological formation in which limestones occur. The oldest rocks are most generally affected, e.g. ”

“ The cellular nature of much of the dolomite has given rise to the term “Zellendolomit.” The Upper Muschelkalk (Hauptmuschelkalk, Friedrichshallkalk of von Alberti) consists of regular beds of shelly limestone alternating with beds of marl. The lower portion or “Trochitenkalk” is often composed entirely of the fragmentary stems of Encrinus liliiformis; higher up come the “Nodosus” beds with Ceratites compresses, C. nodosus, and C. semipartitus in ascending order. In Swabia and Franconia the highest beds are platy dolomites with Tringonodus Sandergensis and the crustacean Bairdia. ”

“ The pebbles and fragments of older rocks which constitute the conglomerate are cemented together by a red or yellow base of dolomite, and in some places the encrinites and other fossils derived from the Mountain Limestone are so detached from the parent rocks that they have the deceptive appearance of belonging to a fauna contemporaneous with the dolomitic beds in which they occur. The imbedded fragments are both rounded and angular, some consisting of sandstone from the coal-measures, being of vast size, and weighing nearly a ton. ”

“ We have already seen (p. 35) how dolomite may undergo striking mineral changes through advanced metamorphic action. Lime-garnets, wollastonite, diopside, and other silicates similarly develop in ordinary limestones exposed to the intrusion of an igneous magma. The extreme changes in such rocks will be described when amphibolites are dealt with. ”

“ Unstable boulders are the great source of danger in this part of the Alps, and even old climbers require to be constantly reminded that on dolomite rocks they must test before they trust every handhold. ”

“ Halfway up the barrier the path splits, and the traveller must either continue to climb steeply and afterwards traverse at a level the higher slopes, or recross the stream and remain in the valley. The upper basin is hemmed in by wooded cliffs, on the top of which lies a ring of pasturages, the base of the dolomite peaks which extend in a complete semicircle round the head of the glen. The sky-line of the range does not equal in boldness or eccentricity of form that of Val di Brenta ”

“ Such a rock is typically exemplified by a coarse-grained sandstone or conglomerate, while a limestone may be naturally porous, or, like the Trenton limestone of Ohio and Indiana, rendered so by its conversion into dolomite and the consequent production of cavities due to shrinkage—a change occurring only in the purer limestones. ”

“ The strata have been eaten away to form valleys, or left standing as
ridges, giving the whole tract a deeply channelled character, the ridges consisting of sandstone and dolomite and the valleys of friable schists. These all trend in the prevalent direction of the Appalachian upheaval, from north-east to south-west. ”

“ Besides the Dolomites included in the above region there are several other Dolomite groups (though less extensive) in the Alps. N.W. of Trent rises the Tosa group, while in Switzerland there are the Piz d’Aela group, S.W. of Bergün on the Albula Pass route, and the curious little group N. of the village of Splügen, besides other isolated peaks between the St Gotthard and Lukmanier Passes. In Dauphiné itself (the home of the geologist Dolomieu) the mountain districts of the Royannais, of the Vercors, and of the Dévoluy (all S.W. of Grenoble) are more or less Dolomitic in character. ”

“ The limestones and dolomites of the upper cretaceous age are also found in various localities mixed with tertiary limestone and sandstone as well as conglomerates, dolomites, and limestones of the lower cretaceous age. In other sections they are mixed with limestones and dolomites of the upper carboniferous age mingled with slate, sandstone and pudding stone. ”

“ Even nowadays there remain many peaks in the Dolomites yet untrodden by the foot of, at least, modern man, as well as numberless delightful paths amid exquisite scenery, where flowers carpet the earth and tiny streams make their water-music. ”

“ Dolomites of compact structure have a higher specific gravity than limestones, but they very often have a cavernous or drusy character, the walls of the hollows being lined with small crystals of dolomite with a pearly lustre and rounded faces. They are also slightly harder, and for these and other reasons they last better as building stones and wear better when used for paving or road-mending. Dolomites are rarely fossiliferous, as the process of dolomitization tends to destroy any organic remains originally present. ”

“ No place on earth will ever draw them back as does the Dolomites, no other can evoke quite the same sensations as when they gaze on those startling contours or catch their breath to see the fiery glow rising over a foreground of blue-black firs. ”

“ When we come to describe the scenery of the Dolomites it is difficult to do it justice. Chief among the weird attractions of these amazing freaks of nature is the curious red colouring which is seen when the sunset lights the high peaks and the valleys lie in blue-grey shadow. ”

“ Section from Monte Calvo to the sea by the valley of Magnan, near Nice.
A. Dolomite and sandstone. (Greensand formation?) a, b, d. Beds of gravel and sand. c. Fine marl and sand of St. Madeleine, with marine shells.
The description above given of the slanting position of the minor layers constituting a single stratum is in certain cases applicable on a much grander scale to masses several hundred feet thick, and many miles in extent. A fine example may be seen at the base of the Maritime Alps near Nice. ”

“ The wild confusion of some parts of the Dolomites, their rent and torn spires, cannot be attributed to volcanic origin, for limestone is not volcanic; the greater part of the gigantic carving must have182 been done by the slow agencies of wind and weather and rain working on silently through age after age, and in some cases by earthquakes, which have overthrown the precariously-balanced needles and solid blocks of stone. ”

“ As compared with limestones they are less frequently well bedded, but there are exceptions to this rule. Many dolomites, particularly those of the north of England, show a very remarkable concretionary structure. The beds look as if made up of rounded balls of all sizes from a foot or two in diameter downwards. Often they are stuck together like piles of shot or bunches of grapes. They are composed of fibrous radiate calcite crystals, which by some kind of concretionary action have segregated from the dolomitic material and grouped themselves together in this way. ”

“ Geography moreover acquires a new interest when we once realise that mountains [Pg 215] are no mere accidents, but that for every mountain chain, for every peak and valley, there is a cause and an explanation.
The origin of Mountains is a question of much interest. The building up of Volcanoes is even now going on before our eyes. Some others, the Dolomites for instance, have been regarded by Richthofen and other geologists as ancient coral islands. ”

“ Crossing this region are many mountain ranges of limestone structure, which by water, weather and other causes have been worn away into the most fantastic fissures and clefts and the most picturesque peaks and pinnacles. A very great charm is their curious coloring, often of great beauty. The region of the Dolomites is a great contrast to the rest of the Alps. Its characteristics do not make the same appeal to all. ”