The 176 m (578 ft) Hill of Howth, overlooking Dublin Bay from the north, seems a likely spot for the nurturing of place-based legends. It was known as the Ben Edair of Fionn MacCumhaill’s army, the Fianna. Muck Rock, the prominence immediately above the Howth Dolmen.
Local lore knew this portal tomb as “Fionn MacCumhaill’s Quoit” [a dolmen, or cromlech, today called a portal tomb]. But nineteenth-century poet and antiquarian Sir Samuel Ferguson believed it to be the grave of the legendary Aideen, who died of grief when her husband Oscar, grandson of Fionn, was slain in battle. Ferguson commemorated the site in his lavishly illuminated poetic work, The Cromlech on Howth, excerpted above.
The weight of this massive block long ago splayed out its uprights and broke its supporting stone; the capstone collapsed at its rear, embedding itself in the ground. When it first attracted the attention of nineteenth-century antiquarians, they argued about whether or not the Cromlech on Howth was actually assembled in that fashion, as a so-called “earth-fast” dolmen.
A dolmen is a type of single-chamber megalithic tomb, usually consisting of two or more vertical megaliths supporting a large flat horizontal capstone or "table". Most date from the early Neolithic (4000–3000 BCE) and were sometimes covered with earth or smaller stones to form a tumulus. Small pad-stones may be wedged between the cap and supporting dolmens are trash stones to achieve a level appearance. In many instances, the covering has weathered away, leaving only the stone "skeleton" of the mound intact.
It remains unclear when, why and by whom the earliest dolmens were made. The oldest known are found in Western Europe, dating from c 7,000 years ago. Archaeologists still do not know who erected these dolmens, which makes it difficult to know why they did it. They are generally all regarded as tombs or burial chambers, despite the absence of clear evidence for this. Human remains, sometimes accompanied by artefacts, have been found in or close to the dolmens which could be scientifically dated using radiocarbon dating. However, it has been impossible to prove that these remains date from the time when the stones were originally set in place.
Quartzite is a hard, non-foliated metamorphic rock which was originally pure quartz sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to grey, though quartzites often occur in various shades of pink and red due to varying amounts of iron oxide (Fe2O3). Other colors, such as yellow, green, blue and orange, are due to other minerals.
When sandstone is cemented to quartzite, the individual quartz grains recrystallize along with the former cementing material to form an interlocking mosaic of quartz crystals. Most or all of the original texture and sedimentary structures of the sandstone are erased by the metamorphism. The grainy, sandpaper-like surface becomes glassy in appearance. Minor amounts of former cementing materials, iron oxide, silica, carbonate and clay, often migrate during recrystallization and metamorphosis. This causes streaks and lenses to form within the quartzite.
Orthoquartzite is a very pure quartz sandstone composed of usually well-rounded quartz grains cemented by silica. Orthoquartzite is often 99% SiO2 with only very minor amounts of iron oxide and trace resistant minerals such as zircon, rutile and magnetite. Although few fossils are normally present, the original texture and sedimentary structures are preserved.
The term is also traditionally used for quartz-cemented quartz arenites, and both usages are found in the literature. The typical distinction between the two (since each is a gradation into the other) is a metamorphic quartzite is so highly cemented, diagenetically altered, and metamorphosed so that it will fracture and break across grain boundaries, not around them.
Quartzite is very resistant to chemical weathering and often forms ridges and resistant hilltops. The nearly pure silica content of the rock provides little material for soil; therefore, the quartzite ridges are often bare or covered only with a very thin layer of soil and little (if any) vegetation.
Quartz is a mineral composed of silicon and oxygen atoms in a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Quartz is the second most abundant mineral in Earth's continental crust, behind feldspar.
Quartz exists in two forms, the normal α-quartz and the high-temperature β-quartz, both of which are chiral. The transformation from α-quartz to β-quartz takes place abruptly at 573 °C (846 K). Since the transformation is accompanied by a significant change in volume, it can easily induce fracturing of ceramics or rocks passing through this temperature threshold.
There are many different varieties of quartz, several of which are semi-precious gemstones. Since antiquity, varieties of quartz have been the most commonly used minerals in the making of jewelry and hardstone carvings, especially in Eurasia.
Fractures in igneous rocks can be the place of circulation and installation of hot aqueous magmas and solutions, with consequent crystallization / precipitation of minerals of varied chemical composition. The filling of the fractures generates more or less tabular bodies, with thicknesses that can be of the order of millimeters or centimeters (forming fillets) up to many meters (veins).
In granites and metasediments, there are layers of basic rocks, quartz and aplito-pegmatite veins and masses. Some of the quartz and aplito-pegmatite veins are mineralized, and in the past there have been tin, tungsten, molybdenum and gold mining operations (such as the former Carris and Borrageiro mines). Unfortunately, much of this geological heritage has been destroyed over the years.