The granites of Dartmoor
Granite with varying grain size; mostly coarse and locally porphyritic. In the north and north-east of the granite, tourmaline is a common constituent, although the amount of tourmaline and biotite varies across the granite; locally one or both minerals may be very abundant. Fine-grained granite occurs in the area around Birch Tor and Vitifer Mine. In the eastern area, the granite has a coarsely crystalline porphyritic texture, composed principally of quartz, biotite and orthoclase but locally with microperthite, albite and black tourmaline. Textures vary over short distances, with the boundaries between them being predominantly gradational. Over the whole extent of the Dartmoor Granite, more than 50% is composed of granite in which feldspar megacrysts of > 15mm compose over 10% of the rock volume. British Geological Survey
Once exposed the granite itself was attacked by the weather, with surprising results. These do not end at the widening of the joints in the rock and the dislodging of fragments from its surface. Acted on by the chemicals in the air, the felspar itself slowly rots and softens. Observer Book of Geology 1968.
It is an undiscovered part of the house. Arriving in the lift the corridor is dim and unfinished with raw white plaster walls, there is gypsum dust everywhere from cut plaster boards. The first door on the right is through a small arch way. It opens into a dark hallway that has not been used for some time, with a dirty, highly patterned Axminster carpet. Another carpet is rolled and propped against the wall. The floor is made from scratched bare pine floor boards. Dilapidated furniture and other junk partially blocks the entrance to an open door on the right through which can be see a bathroom in a state of disrepair, with the avocado washstand and basin partially dismantled. Some natural light comes into the hallway though a window in the bathroom. A short way down the hall the refurbishments have been mostly completed and it is like a completely different house, flooded with light. The space opens out and upwards into a large split level apartment with floor to ceiling windows which show a panorama of the old city rooftops, punctuated by towers and spires of shelliflorous limestone, blue slate, green copper and oyster lead. Extract from One Thousand Stones
Photo: Structural and Field Geology, James Geikie (1905)