Basalt with glassy magma mainly devitrified. Brent Tor, North-west side of the foot of the Tor, Magnification x55.
The 'Eruptive rocks of Brent Tor and its neigbourhood' was the first special petrographical work issued by the Geological Survey. It is dated 1878. It denotes a time when microscopic examination of rocks collected during the mapping of new districts became routine. Colour chromolithographs.
Frank Rutley was also the author of Elements of Mineralogy first published in 1874 and still available in print today in its 27th edition!
1. Decomposed Greenstone, Greston Bridge.
2. Slate beneath lower Greenstone, Greston Bridge.
3. Elvan, S. end of Shilla Mill Quarry (Polarized).
4.Mag'n. Mica and cavities in Quartz of Granite, Brazen Tor.
5. Fluid cavities containing cubes of NaCl, Brazen Tor.
6. Amygdaloidal Schalstein, Churlhanger, near Lamerton.
No. 1 Hornblende, Pyrites and Quartz in Amphibolite, Brazen Tor, Devon
No 2 Schorl in the Granulitic margin of the Granite in contact with No. 1
No 3. Pyroxene (probably Diallage) in Gabbro? Cottage Inn, Main Road to Marytavy.
No. 4 Titaniferous Iron, partly altered (Gabbro) Cock's Tor, near Tavistock.
Bo. 1 x55. Nos. 2,3 and 4 x25. ordinary transmitted light. On the surface of No. 4 a little light was also reflected simultaneously.
Brent Tor. (South side near the base) x25
Brent Tor. (North side) x25
A selection of plates from the memoir:
Brent Tor from Hart's Hall, near Morwellham
The Master Rock, Horndon. (woodcut)
Posted by Bob McIntosh