Tuesday, 16 December 2014

Geological report on the Faldouet Dolmen

Today's piece is from a trawl through the archives. It is an article by Dr Arthur Mourant, published in 1932. It is a companion piece to that on the archaeology and history of the dolmen published by Major Rybot, which you can read here.


Rocks - Some Notes on Terminology.

A few notes on the geological terms used by Dr Mourant

The term 'granite' applies to a group of intrusive igneous rocks with similar textures and slight variations on composition and origin. These rocks mainly consist of feldspar, quartz, mica, Intrusive rocks are where magma gradually pushes up from deep within the earth into cracks or spaces or pushing existing rocks aside. Because it forms deep underground, these rocks are often known as plutonic rocks, after Pluto, the god of the underworld.

As the rock slowly cools into a solid, the different parts of the magma crystallize into minerals. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. Mica is the black in granite, feldspar the pink, and quartz the white.

A granitic rock with a porphyritic texture is known as a granite porphyry. Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group. Some of the rocks at Faldouet dolmen are of this type.

The extrusive igneous rock equivalent of granite is rhyolite, which Dr Mourant also notes as present at Faldouet and the surrounding coastline.

Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, and consequently, outcrops of rhyolite may bear a resemblance to granite. Due to their high content of silica and low iron and magnesium contents, rhyolite melts are highly polymerized and form highly viscous lavas

Extrusive refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics. This is as opposed to intrusive rock formation such as granite, in which magma does not reach the surface.

Porphyritic rocks are formed when a column of rising magma is cooled in two stages. In the first stage, the magma is cooled slowly deep in the crust, creating the large crystal grains, with a diameter of 2 mm or more. In the final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano, creating small grains that are usually invisible to the unaided eye.

Geological report on the Faldouet Dolmen
By Dr. A. E. Mourant.

The uprights, with four exceptions, are composed of dark red granite of the Mont Orgueil type. There is one upright each of coarse porphyritic granite slightly porphyritic granite (No. 3), granite porphyry (No. 23) and diorite (No. 22),

The cap-stone (No. 52) consists of brecciated non-porphyritic flow-rhyolite.The Mont Orgueil granite is a very well-defined type, only known at Mont Orgueil and on the foreshore for less than half a mile to the south-west. Blocks of this rock now in the Dolmen must have been carried about a quarter of a mile, and raised at least 130 feet.

The nearest known diorite is about a mile south-west of the dolmen, at the edge of the high ground on which it stands. The coarse porphyritic granite probably came from the foreshore, over a mile and a half to the south, and the slightly porphyritic granite from the high ground the same distance away to the south-west. Granite porphyry is known at a number of places a short distance to the south of the dolmen.

The country rock underlying the dolmen and out-cropping near it is rhyolite, but it differs essentially from the materials of the cap-stone in being porphyritic. The nearest outcrop of non-porphyritic rhyolite is a quarter of a mile to the north, almost on a level with the dolmen. It will be seen that, while the lighter uprights were brought uphill, the massive capstone was probably moved along level ground.

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