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This is a digitized version of an article from The Times's print archive, before the start That is Humphrey Cobb's "Paths of Glory," a shocking story of a shameful.
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Click here to see more Tap here to see more Tap here to see more. Accessibility Links Skip to content. Log in Subscribe. Read the full article. Start your free trial. In general, the mineral assemblage preserved in a metamorphic rock is frozen at the highest temperature experienced during metamorphism see above Retrograde metamorphism , and thus the facies and facies series to which the rock would be assigned reflect only a single point on its P-T-t path. Because these paths are so strongly linked to dynamic processes, their reconstruction provides a means by which tectonic processes operative in the geologic past may be understood.

Owing to the continuous recrystallization of rocks that occurs during progressive metamorphism, much of the early record of metamorphic changes within a sample is eradicated by later events. It is, therefore, not possible to determine the entire P-T-t path followed by an individual sample, but often enough disequilibrium features are preserved to permit reconstruction of a few thousand bars and a couple of hundred degrees of the path; such a portion may represent anywhere from a few million to a hundred million years of Earth history, as revealed by geochronologic determinations involving different minerals or fabric generations in the sample.

Techniques for determining the pressure-temperature history of a metamorphic rock include using compositions of coexisting minerals to calculate pressures and temperatures of equilibration geobarometry and geothermometry, respectively , comparing the mineral assemblage to experimentally determined stability fields for the phases, utilizing mineral inclusions enclosed within porphyroblasts to constrain assemblages present in the early history of the sample, and making use of the densities of small inclusions of fluids trapped within the minerals to determine possible pressures and temperatures experienced at different stages in the burial and uplift history.

It is convenient to distinguish several general types of metamorphism in order to simplify the description of the various metamorphic phenomena. Recognized here are hydrothermal, dynamic, contact, and regional metamorphism, each of which will be described in turn.

We discuss an algorithm to find a minimum path cover on a DAG.

Changes that occur in rocks near the surface where there is intense activity of hot water are categorized as hydrothermal metamorphism. It is now generally recognized that the circulating groundwaters that often become heated by their proximity to igneous materials produce the metamorphism. Migration of chemical elements , vein formation, and other kinds of mineral concentration may be extreme on account of the large volumes of water circulated. When directed pressure or stress is the dominant agent of metamorphism, it is termed dynamic; other terms are dislocation, kinematic, and mechanical metamorphism.

Mineralogical changes occurring on a fault plane provide an obvious example. In some such cases, the action may simply be a grinding up of existing grains or realignment of minerals that have non-equant crystals.


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If the action is intense, friction may even lead to melting. Whenever silicate melts magmas , from which igneous rocks crystallize within Earth invade the crust at any level, they perturb the normal thermal regime and cause a heat increase in the vicinity. If a mass of basaltic liquid ascending from the upper mantle is trapped in the crust and crystallizes there, it will heat the surrounding area; the amount of heating and its duration will be a direct function of the mass and shape of the igneous material.

Contact-metamorphic phenomena thus occur in the vicinity of hot igneous materials and at any depth. Under such circumstances, pressure and temperature are not simply correlated. Thermal gradients are often steep unless the igneous mass is extremely large.

Paths of Glory - Wikipedia

Contact aureoles —the surrounding zones of rock that become altered or metamorphosed—vary in thickness from several centimetres around tabular bodies such as dikes and thin sills to several kilometres around large granitic intrusions. The contact metamorphic rocks of the aureole zone often lack any obvious schistosity or foliation.

The facies associated with contact metamorphism include the sanidinite, pyroxenite - hornfels , hornblende -hornfels, and albite - epidote -hornfels facies. Rocks of the sanidinite facies are represented by small fragments of aureole materials that have often been totally immersed in silicate liquids or by the aureole rocks surrounding volcanic pipes. Very high temperatures are attained, often at very low pressures.


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    4. The dominant feature of the mineralogy of this facies is an almost complete lack of minerals containing water or carbon dioxide. Many of the minerals show similarity to those of igneous rocks themselves. If the duration of heating is short, adjustment to the imposed temperature is often imperfect. Pelitic rocks high in aluminum oxide contain minerals such as mullite , sillimanite , sanidine, cordierite , spinel , hypersthene, anorthite , tridymite , and even glass.

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    One of the classic localities of such rocks is the island of Mull , off the west coast of Scotland , but these rocks can be found in most regions of volcanism. Calcareous rocks originally impure limestones or dolomites tend to lose nearly all their carbon dioxide, but pure calcite may survive.

    Solving Minimum Path Cover on a DAG

    Typical metamorphic minerals are quartz , wollastonite , anorthite, diopside , periclase , and in some places the classic is Scawt Hill in Northern Ireland an array of complex calcium silicates such as spurrite, larnite, rankinite, melilite , merwinite, and monticellite. These minerals result from the addition of varying amounts of silica to impure mixtures of calcite and dolomite. In a general way the minerals of this facies are reminiscent of those of industrial slags. Rocks of the pyroxene - hornfels facies are characteristically formed near larger granitic granite or gabbroic gabbro bodies at depths of a few kilometres or at pressures of a few hundred bars.

    The mineral assemblages are again largely anhydrous, but, unlike the sanidinite facies , the minerals reflect distinctly lower temperatures.