This is a name given to those varieties of what would otherwise be diorite and gabbro which are too fine-grained in character to enable one to tell whether the dark-colored mineral which they contain is hornblende or pyroxene without the use of a microscope. The dolerites are dark-colored rocks occurring in the form of small intrusive bosses, small laccoliths, sills and dykes. They are essentially holocrystalline aggregates of plagioclase and monoclinic augite; but various mineralogical types are furnished by the additional associations of a rhombic pyroxene, hornblende, mica, olivine or quartz, as for example hypersthene dolerite, hornblende dolerite, mica dolerite, olivine-dolerite, quartz-dolerite. A variety consisting almost entirely of plagioclase has been termed leucophyre. Owing to the equal or predominant amount of ferromagnesian minerals, the color of these rocks is dark, medium or dark gray or greenish to black. As in most rocks the tone of color is best observed in viewing the rock at a little distance, so that the individual grains become indistinguishable and only their mass effect is seen. Many dolerites have a green colour due to the presence of a member of the chlorite family, which is developed at the expense of the augite. This gives rise to the name greenstone, which is still used as a handy field term without regard to composition. This term as here used comprises not only the finer-grained diorites and gabbros but much also of what is termed "diabase" by the petrographers, as well as occasional rare rocks which need no mention here. The term diabase has been used in place of dolerite, especially in the US. Elsewhere its use is not to be commended, since in Germany it has been applied to the more altered (chloritized) types, including those of volcanic origin.
The plagioclase of the dolerites is labradorite or anorthite; it may be quite fresh, or much decomposed, altering in the latter case to an aggregate of minute granules of epidote, calcite, and occasionally of quartz. In thin section the augite is usually of a pale violet brown tint. It mostly occurs in large ophitic masses, enclosing lath-shaped crystals and microlites of feldspar. In altered rocks the augite is partially, sometimes even completely, replaced by a scaly and fibrous aggregate of some chloritic mineral. The original hornblende of the hornblende-dolerites is usually the common brown variety. In the so-called epidiorites, however, it is of a pale green color (uralite, actinolite, smaragdite), and is produced by paramorphism of the augite. The olivine of the olivine-bearing dolerites is rarely quite fresh, being stained brown with limonite, blackened with magnetite dust, or completely replaced by serpentine or calcite. When quartz occurs in the dolerites it usually forms a micrographic intergrowth with feldspar, and the resulting micro-pegmatite is interstitial between the other constituents (e.g. in dolerites of the Ratho type among the Carboniferous Rocks of Scotland). Such rocks are intimately related to tne dioritic granophyres or markfieldites. Among accessory minerals the iron-ores, magnetite and ilmenite are always more or less abundantly present, the latter giving rise, when altered, to leucoxene (an amorphous variety of sphene). Pyrites are sometimes present in disseminated granules, and apatite in six-sided needles. Quartz, calcite, epidote, chlorite and serpentine are common secondary minerals; and these minerals are often distributed in veins and nests through the rocks. The structure of the dolerites is characteristically ophitic; but in some cases a granular structure prevails.
This is due to the occurrence of the augite in round granules, instead of in large ophitic plates enveloping the lath-shaped feldspars. Where porphyritic structure is developed, phenocrysts of feldspars, and sometimes of olivine and augite, are embedded in a groundmass of feldspar microlites and granular augite. Interstitial glassy matter is only found as a marginal phenomenon. The Carboniferous intrusions of the Midland Valley of Scotland afford excellent examples of nearly all the varied types of structure that have been met with in the dolerites. The intrusive character of the dolerites is especially shown by their contact phenomena: baking or "porcellanization" of adjoining shales: formation of banded and spotted rocks (the so-called desmoisites and spilosites), and of hornstone-like rocks rich in alkalies (adinoles). The texture of these rocks is fine grained granular, they are sometimes porphyritic but these cases are described in the following section on the porphyries. Their chemical composition, in the great majority of cases, is similar to that of the diorites and gabbros already given and need not be repeated. They are heavy, the specific gravity being from 3.0-3.3. Their jointing is usually small cuboidal, wedge shaped or platy, often columnar and sometimes on a very large scale, though generally this structure is not so perfect as in the finer-grained basalt. It is most apt to occur in dikes, very thick intrusive sheets and in massive extrusive flows.
The dolerites do not occur in large stocks and batholith like the diorites and gabbros, though not infrequently these latter rocks pass into an endomorphic phase of dolerite at the margin of the intrusion. As intrusive rocks they belong in the minor class, being found in dikes, small laccoliths and intrusive sheets, the latter often of great thickness, and in thick massive lava flows whose cooling has been slow.
In the eastern United States the most conspicuous examples are found in the intrusions and flows of "trap" of the Triassic formations stretching from Nova Scotia to Georgia. Through faulting and erosion they now give rise to definite topographic features, such as the ridges in Connecticut and the Palisades opposite New York City. Similar masses of these rocks are found in the Lake Superior region and in the great lava flows of the western United States. In all these occurrences they are associated with, and pass into, the denser forms of basalt. These larger occurrences of dolerite mostly contain pyroxene as the dominant ferromagnesian mineral and are largely the rock called "diabase" by petrographers, while cases where hornblende is dominant are mostly confined to dikes and smaller intrusions, especially in the older rocks.
Dolerites are also very common rocks in Great Britain in various localities, in dikes and intrusive sheets, and especially in the north of Scotland and Ireland where they are often extrusive and associated with denser basalts. They are in fact very common rocks in all parts of the world. From what has been said it is easy to see that the dolerites are a class of rocks based largely on convenience. On the one hand they form a transition group, based on texture, between the diorites and gabbros and the dense basalts, and on the other they cannot depend wholly on texture, because relatively coarse-grained rocks may occur in which one cannot distinguish between hornblende and pyroxene and which must therefore be placed in this class.
The case might occur in which, instead of hornblende or pyroxene, biotite was the dominant mineral associated with the feldspar. Such rocks are not very common but sometimes occur, especially in dikes and sheets and with quite fine grain. They form the rocks called mica trap or minette, mentioned later under basalt The pyroxenic members of this group, by regional metamorphism, become converted into hornblende rocks, generally into hornblende schist, and both varieties by alteration may produce chlorite and pass into the so-called "greenstones." These alterations are quite similar to what has been described under gabbro. By weathering they become brownish and discolored and ultimately yield brown ferrugineous soils.
Uses. The rocks of this group are too dark and somber for general use in fine architectural or interior work, except for certain monumental purposes. The "trap" of the eastern states has been considerably employed in rough masonry, and where good natural joint faces can be used for wall surfaces, the brown weathering color gives pleasing effects. The toughness of the material, which the traps afford, has however caused it to be considerably used for block paving and the crushed stone for road making.
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