Porphyry (geology) in the context of "Rhyolite"

In modern scholarship, the "late" period of the Roman army begins with the accession of the Emperor Diocletian in AD 284, and ends in 480 with the death of Julius Nepos, being roughly coterminous with the Dominate. During the period 395–476, the army of the Roman Empire's western half progressively disintegrated, while its counterpart in the East, known as the East Roman army (or the early Byzantine army) remained largely intact in size and structure until the reign of Justinian I (r. AD 527–565).

The Imperial Roman army of the Principate (30 BC – 284 AD) underwent a significant transformation as a result of the chaotic 3rd century. Unlike the army of the Principate, the army of the 4th century was heavily dependent on conscription and its soldiers were paid much less than in the 2nd century. Barbarians from outside the empire probably supplied a much larger proportion of the late army's recruits than in the army of the 1st and 2nd centuries, but there is little evidence that this adversely affected the army's combat performance.

A cairn is a human-made pile (or stack) of stones raised for a purpose, usually as a marker or as a burial mound. The word cairn comes from the Irish: carn [ˈkʰaːrˠn̪ˠ] (plural cairn [ˈkʰaːrˠɲ]).

Cairns have been and are used for a broad variety of purposes. In prehistory, they were raised as markers, as memorials and as burial monuments (some of which contained chambers).

In the burial practices of ancient Rome and Roman funerary art, marble and limestone sarcophagi elaborately carved in relief were characteristic of elite inhumation burials from the 2nd to the 4th centuries AD. At least 10,000 Roman sarcophagi have survived, with fragments possibly representing as many as 20,000. Although mythological scenes have been quite widely studied, sarcophagus relief has been called the "richest single source of Roman iconography," and may also depict the deceased's occupation or life course, military scenes, and other subject matter. The same workshops produced sarcophagi with Jewish or Christian imagery. Early Christian sarcophagi produced from the late 3rd century onwards, represent the earliest form of large Christian sculpture, and are important for the study of Early Christian art.

They were mostly made in a few major cities, including Rome and Athens, which exported them to other cities. Elsewhere the stela gravestone remained more common. They were always a very expensive form reserved for the elite, and especially so in the relatively few very elaborately carved examples; most were always relatively plain, with inscriptions, or symbols such as garlands. Sarcophagi divide into a number of styles, by the producing area. "Roman" ones were made to rest against a wall, and one side was left uncarved, while "Attic" and other types were carved on all four sides; but the short sides were generally less elaborately decorated in both types.

Pompey's Pillar (Arabic: عمود السواري, romanized: 'Amud El-Sawari) is a Roman triumphal column in Alexandria, Egypt. Despite its modern name, it was actually set up in honour of the Roman emperor Diocletian between 298–302 AD. The giant Corinthian column originally supported a colossal porphyry statue of the emperor in armour. It stands at the eastern side of the temenos of the Serapeum of Alexandria, which is now in ruins.

It is the only ancient monument in Alexandria that is still standing in its original location.

The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystals, or clasts are embedded.

The matrix of an igneous rock consists of finer-grained, often microscopic, crystals in which larger crystals, called phenocrysts, are embedded. This porphyritic texture is indicative of multi-stage cooling of magma. For example, porphyritic andesite will have large phenocrysts of plagioclase in a fine-grained matrix. Also in South Africa, diamonds are often mined from a matrix of weathered clay-like rock (kimberlite) called "yellow ground".

A subvolcanic rock, also known as a hypabyssal rock, is an intrusive igneous rock that is emplaced at depths less than 2 km (1.2 mi) within the crust, and has intermediate grain size and often porphyritic texture between that of volcanic rocks, which are extrusive igneous rocks, and plutonic rocks, which form much deeper in the ground. Subvolcanic rocks include diabase (also known as dolerite) and porphyry. Common examples of subvolcanic rocks are diabase, quartz dolerite, microgranite, and diorite.

A phenocryst is an early forming, relatively large and usually conspicuous crystal distinctly larger than the grains of the rock groundmass of an igneous rock. Such rocks that have a distinct difference in the size of the crystals are called porphyries, and the adjective porphyritic is used to describe them. Phenocrysts often have euhedral forms, either due to early growth within a magma, or by post-emplacement recrystallization. Normally the term phenocryst is not used unless the crystals are directly observable, which is sometimes stated as greater than 0.5 mm (0.020 in) in diameter. Phenocrysts below this level, but still larger than the groundmass crystals, are termed microphenocrysts. Very large phenocrysts are termed megaphenocrysts. Some rocks contain both microphenocrysts and megaphenocrysts. In metamorphic rocks, crystals similar to phenocrysts are called porphyroblasts.

Phenocrysts are more often found in the lighter (higher silica) igneous rocks such as felsites and andesites, although they occur throughout the igneous spectrum including in the ultramafics. The largest crystals found in some pegmatites are often phenocrysts being significantly larger than the other minerals.