I can't believe this is still going!!
Tonight's numbers are
16. Greywacke
Greywacke is a variety of sandstone generally characterized by its hardness, dark colour, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix.
The origin of greywacke was problematic until turbidity currents and turbidites were understood, since, according to the normal laws of sedimentation, gravel, sand and mud should not be laid down together. Geologists now attribute its formation to submarine avalanches or strong turbidity currents. These actions churn sediment and cause mixed-sediment slurries, in which the rocks may exhibit a variety of sedimentary features. Supporting the turbidity current origin theory is that deposits of greywacke are found on the edges of the continental shelves, at the bottoms of oceanic trenches, and at the bases of mountain formational areas. They also occur in association with black shales of deep sea origin.
Greywackes are mostly grey, brown, yellow or black, dull-coloured sandy rocks which may occur in thick or thin beds along with slates and limestones.
They are abundant in Wales, the south of Scotland, the Longford Massif in Ireland and the English Lake District and compose the majority of the main alps that make up the backbone of New Zealand. They can contain a very great variety of minerals, the principal ones being quartz, orthoclase and plagioclase feldspars, calcite, iron oxides and graphitic, carbonaceous matters, together with (in the coarser kinds) fragments of such rocks as felsite, chert, slate, gneiss, various schists, and quartzite. Among other minerals found in them are biotite, chlorite, tourmaline, epidote, apatite, garnet, hornblende, augite, sphene and pyrites. The cementing material may be siliceous (sandy) or argillaceous (clayey) and is sometimes calcareous (like limestone).
As a rule greywackes are not fossiliferous, but organic remains may be common in the finer beds associated with them. Their component particles are usually not very rounded or polished, and the rocks have often been considerably indurated by recrystallization, such as the introduction of interstitial silica.
20. Ignimbrite
I have a particular affection for Ignimbrites, as they are present as a rare rock type in the Late Ordovician (450 million years ago) Borrowdale Volcanic Group of the Lake District, which I studied as part of my undergraduate mapping project approximately 1 million years ago.
An ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow. It is a hot suspension of particles and gases flowing rapidly from a volcano driven by having a greater density than the surrounding atmosphere. New Zealand geologist Patrick Marshall derived the term 'ignimbrite' from ‘fiery rock dust cloud’ (from the Latin igni- (fire) and imbri- (rain)), formed as the result of immense explosions of pyroclastic ash, lapilli and blocks flowing down the sides of volcanoes.
Ignimbrites are made of a very poorly sorted mixture of volcanic ash (or tuff when lithified) and pumice lapilli, commonly with scattered lithic fragments. The ash is composed of glass shards and crystal fragments. Ignimbrites may be loose and unconsolidated or lithified (solidified) rock called lapilli-tuff. Proximal to the volcanic source, ignimbrites commonly contain thick accumulations of lithic blocks, and distally, many show metre thick accumulations of rounded cobbles of pumice.
Ignimbrites may be white, grey, pink, beige, brown or black depending on their composition and density.
Ignimbrite is primarily composed of a matrix of volcanic ash (tephra) which is composed of shards and fragments of volcanic glass, pumice fragments, and crystals. The crystal fragments are commonly blown apart by the explosive eruption. Most are phenocrysts that grew in the magma, but some may be exotic crystals such as xenocrysts, derived from other magmas, igneous rocks, or from country (surrounding) rock.
The ash matrix typically contains varying amounts of pea- to cobble-sized rock fragments called lithic inclusions. They are mostly bits of older solidified volcanic debris entrained from conduit walls or from the land surface. More rarely, clasts are cognate material from the magma chamber.
If sufficiently hot when deposited, the particles in an ignimbrite may weld together, and the deposit is transformed into a 'welded ignimbrite', made of eutaxitic lapilli-tuff. When this happens, the pumice lapilli commonly flatten, and these appear on rock surfaces as dark lens shapes, known as fiamme.
Welded ignimbrite displaying fiamme
Large hot ignimbrites can create some form of hydrothermal activity as they tend to blanket the wet soil and bury watercourses and rivers. The water from such substrates will exist in the ignimbrite blanket in fumaroles, geysers and the like, a process which may take several years, for example after the Novarupta tuff eruption. In the process of boiling off this water, the ignimbrite layer may become metasomatised (altered). This tends to form chimneys and pockets of kaolin-altered rock.
Ignimbrites form sheets that can cover as much as thousands of square kilometers. Some examples create thick, valley-filling deposits, while others form a landscape-mantling veneer that locally thickens in valleys and other palaeotopographic depressions.
Often, but not always, a caldera will form as a result of a large ignimbrite eruption because the magma chamber underneath will drain and thus can no longer support the weight of the rock above.
Ignimbrite occurs very commonly around the lower Hunter region of the Australian state of New South Wales. The ignimbrite quarried in the Hunter region at locations such as Martins Creek, Brandy Hill, Seaham (Boral) and at the now disused quarry at Raymond Terrace is a volcanic sedimentation rock of Carboniferous age (280-345 million years). It had an extremely violent origin. This material built up to considerable depth and must have taken years to cool down completely. In the process the materials that made up this mixture fused together into a very tough rock of medium density.
Ignimbrite also occurs in the Coromandel region of New Zealand, where the striking, orange-brown ignimbrite cliffs form a distinctive feature of the landscape. The nearby Taupo Volcanic Zone is covered in extensive, flat sheets of ignimbrite erupted from caldera volcanoes during the Pleistocene and Holocene. The exposed ignimbrite cliffs at Hinuera (Waikato) mark the edges of the ancient Waikato River course which flowed through the valley prior to the last major Taupo eruption 1800 years ago. The west cliffs are quarried to obtain blocks of Hinuera Stone, the name given to welded Ignimbite used for building cladding. The stone is light grey with traces of green and is slightly porous.
Hinuera stone carved into a fireplace
Huge deposits of ignimbrite form large parts of the Sierra Madre Occidental in western Mexico. In the western U.S., massive ignimbrite deposits up to several hundred metres thick occur in the Basin and Range Province, largely in Nevada, western Utah, southern Arizona, and north-central and southern New Mexico, and Snake River Plain. The magmatism in the Basin and Range Province included a massive flare-up of ignimbrite which began about 40 million years ago and largely ended 25 million years ago: the magmatism followed the end of the Laramide orogeny (mountain building episode which formed the Rocky Mountains 70 - 40million years ago), when deformation and magmatism occurred far east of the plate boundary. Additional eruptions of ignimbrite continued in Nevada until roughly 14 million years ago. Individual eruptions were often enormous, sometimes up to thousands of cubic kilometres in volume, giving them a Volcanic Explosivity Index of 8, comparable to Yellowstone Caldera and Lake Toba eruptions.
Yucca Mountain Repository, a U.S. Department of Energy terminal storage facility for spent nuclear reactor and other radioactive waste, is in a deposit of ignimbrite and tuff.
The layering of ignimbrites is utilized when the stone is worked, as it sometimes splits into convenient slabs, useful for flagstones and in garden edge landscaping.
In the Hunter region of New South Wales ignimbrite serves as an excellent aggregate or 'blue metal' for road surfacing and construction purposes.
I picked the coprolite for JUST the reason you posted a warning! 
I knew what it was and it's always a bit entertaining to see the reaction people have to finding out what it is. So amazing that you can find out EXACTLY what people were eating thousands of years ago sometimes.
I'm finding all this info interesting.
I came to just see how the bingo was going and read all the interesting info, and then I remembered I had signed up! 
(I'm still at 3/10 but am learning a lot!)
