Kerbstones outside Edgware Road Metropolitan Station.

The kerbstones include several kinds of granite. The curved corners are of grey Cornish Granite, with enormous orthoclase crystals (see detail). One stone has a large dark-grey inclusion of some slate-like material which the molten granite engulfed deep down, perhaps four or five kilometres under the surface, as it melted its way upwards. The granite, because it did not reach the surface, cooled very slowly indeed and had time to form large crystals of feldspar. As we saw with the Red Swedish Granite of the Green Man, in Bell St, the feldspar crystals formed first, while the surrounding material was still pasty. Therefore they were free to assume their perfect crystal shapes.


Four Types of Orthoclase Feldspar Crystals which
are present in Cornish Granite. Fig. 118.

Above Two kinds of simple orthoclase crystals.
Below Two kinds of twinned orthoclase crystals.


The crystals have three unequal axes, two of which cross at right angles. The third axis is oblique to the plane of the others.

(Ortho - Gk for straight, right; Clace - Gk for break off, in pieces)

Orthoclase means that the crystals split (cleave) in two directions, at right angles to each other.

Crystal forms

Monoclinic

Hexagonal

Feldspar Quartz

a vertical axis
b horizontal axis
c axis tilted to the others

c vertical axis
a1 a2 a3 three equal horizontal axes.

The way crystals split.  


Curved Granite corner kerbs

The Cornish mines specialized in the manufacture of these curved kerbs so that almost all the street corners have light grey Cornish Granite corners which are followed immediately by a variety of other stones, as we saw in Bell Street.

25 Chapel Street

This 1959 building is typical of many built at the time.

 

The Granite Steps of 25 Chapel Street

These are of light grey granite . with dark shreds of biotite and some white lath-shaped feldspar crystals which show up clearly on the top surface.


The Foyer

The foyer of 25 Chapel Street is of polished white marble with grey-blue streaks. This is a typical Carrara Marble, from Italy, popular for panelling and fishmongers' slabs. It used to be called Sicilian Marble, because it was brought from Italy by boats which also carried orange and lemons from Sicily.
Sicilian Marble in the foyer of
25 Chapel Street, Edgware Road.

Detail of the marking on the Sicilian Marble
The firm importing it wanted to put its competitors off the scent, so they spread the rumour that the marble came from Sicily too. Other firms searched the island but to no avail.
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Updated: August 16, 2011