Corals in Chapel Street

Limestones can be grouped into :-

  • Shelly Limestones containing chalky shells of animals long dead.
  • Crinoidal Limestones like the M&S wall, containing the `arms' and stems of Crinoids, the sea lilies (krinon Gk krinoedes - lily)
  • Foraminiferan Limestones containing the shells of microscopic marine animals  called foraminifera.
  • Oolitic Limestone (fish egg limestone). Here the calcium carbonate has been chemically precipitated as small spherical (oolitic) grains which look like fish eggs. These limestones are not made from the shells of dead creatures but are formed by chemical precipitation when dissolved calcium becomes solid. This happens only in shallow water. Deep water, which is under greater pressure because of the weight of water above it, can contain more dissolved calcium carbonate than can shallow water. This causes the oolites formed in shallow water, to dissolve again if they sink too far.

A "Sugar-coated" Marble Wall

At the left hand side of the entrance is a recess with a platform about half a metre above the ground. The right hand end of this has a white, sugary surface. Grains which have fallen off can be found below. A lens reveals sharp-edged grains of irregular shape, not cubic as granulated sugar would be. This was the same Sicilian Marble as in the foyer, but the surface had been weathered until it powdered off.

Serpentinite

Below the platform and running aound the building, is a rich green 5erpentinite plinth with flecks in it.

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The Edgware Road Entrance to the Metropolitan Railway

Until 1987 the entrance to the Metropolitan Railway nearby was panelled with handsome slabs of Serpentinite. This had been the characteristic decoration of the line from 1863. Above were beautiful tiles, not glazed but made of glass fragments just melted enough to hold together and so offering a multitude of reflecting surfaces within their thickness, so that the glaze went the full depth of the tile, giving an appearance unobtainable in any other way. The Martin Brothers achieved a similar depth in their ware, with a hard, metallic glitter, glassy and slightly menacing. Unfortunately this irreplaceable tiling and the fine Serpentinite, have given way to lavatory tiles. The Serpentinite needed a simple cleaning, while the tiles were perfect. One could weep. They will cover Nelson's Column with Formica next.

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The Serpentinite, of which a few fragments remain at the entrance to the station, comes from the deeper levels of mountain ranges, where it has been under great strain. This dark green variety came from the region of Savoy, in Italy, north of Turin. It is often called Verde Antiquo. Over the ages the rock of the Underground Station was shattered by earth movements and the cracks slowly filled with cream quartz. The rock was cracked, filled, re-cracked and refilled several times. Cracks can be seen crossing earlier cracks. This pattern gives a close impression of a serpent's skin which gives the stone its name.

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The Serpentinite at 25 Chapel St tells a different story. There are no signs of cracking, showing that the stone has lived a peaceful life, free from great strains and torment.

The Three Great Classes of Stone

Standing in the gap between the M&S and 25 Chapel Street you have:-

Igneous rock on one side (the granite steps)

 

Sedimentary rocks on the other (the crinoidal, limestone) and

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Metamorphic rocks (marble and serpentinite below).

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The Vertical Panels between the windows of the M&S Building



Pietra Persichina di Prun from
Marks & Spencer's windows.

The fossil gastropods can be seen clearly, as can the lines of faulting which show up as irregular grooves across the surface.

Italian orange-pink `marble' which still contains fossils of gastropods (snails). It is. probably Pietrta Persichina di Prun, from Verona. It is of Cretaceous Age (Gk creta chalk approximately 72m.y. from 136 to 64 m.y. years ago). The fossils have not been melted and recrystallized (metamorphosed) so it is still technically a limestone and not a fully developed marble. We saw strips of Formica at the entrance to 25 Chapel Street, made from a photograph of Botticcino Marble. The fossils in this marble had been dissolved by heat and pressure, so their shapes were lost and the material recrystallized as patches of marble. This has not happened to the Petra Persichina and we can still recognise the fossils.

The orange colour is distinctive, as are the solution effects (the undulating, breaking lines, roughly parallel to each other, crossing the slabs). These were caused by pressure and can make the marble delicate to handle. Some firms now support their marbles with plastic which strengthens them. It also allows thinner veneers to be cut, so it is more economical. This stone has been used in its natural form, but some of the cracks appear to have been filled with a pink grouting.

This description of the Pietra Persichina di Prun was written in 1989 for the second edition of the book. Since the the polluted London air had taken its toll. The brilliant pink has faded and the colours have become drab.

 

The Square Pillars at the doors of Marks & Spencer's

These pillars have orange Pietra Persichina di Prun on two sides and Black Gabbro, called by the trade "Black Granite", on the others. It is coarse grained and is dark because it lacks the light coloured quartz that is found in real granite.

Many colours can be seen in the depths of the Gabbro. There is grey feldspar, black olivine, gold biotite, while sparkling fragments of metals such as platinum and copper may be present.

 

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Updated: October 25, 2011