

PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 223 
From interbedded Porphyry. 
17. The quartzite which caps the peaked summit of Canisp, in Sutherland, 
contains great beds of porphyry, highly characteristic from the rapidity of 
their decomposition, and the spongy clay which is the result of that decom- 
position. 
Banks of this clay spot the south-eastern slope, just below the summit; 
and from out of this clay, brick-red, well-formed crystals of orthoclase may be 
picked in numbers, along with less frequent and smaller crystals of ochre- 
yellow to cream-coloured oligoclase. The crystals of orthoclase are of all sizes, 
from that of a pea to one inch. Their specific gravity is 2° 245. 
1°3 grammes yielded— 
Silica, . , . * 825 
From Alumina, ; - 001 
° 826 = 63 * 538 
Alumina, . . : 17° 363 
Ferric Oxide, : 5 1: 867 
Manganous Oxide, : 384 
Lime, . 2 : : 17335 
Potash, : : : 12 : 932 
Soda, . : f ; Le 6e5 
Water, in 123, 
100 + 237 
Loss in bath, ‘395 p.c. Insoluble silica, 1°573 p.c.; possible impurity, 
oligoclase. 
From Trap Tufa. 
18. Large twin crystals of glassy felspar occur somewhat rarely imbedded in 
the tuff of Kinkell, near St Andrews. These crystals are sometimes about two 
inches in size, generally about one inch. They are invariably twins, and also 
invariably much worn, rounded, and smoothed on the angles, preserving only 
tudely the form of crystals. They lie in the loose friable tuff in no special 
direction, and in no way in connection with either drusy cavity, exfiltration vein, 
or with any other mineral. The wearing is neither like that of water or sand 
friction ; they have more the appearance of a portion of their substance having 
been dissolved away. ‘They are quite unaltered, fresh-looking, of a brilliant 
lustre, transparent, but sometimes much fissured in the interior. 
To the writer the position of these crystals is an enigma. Colour, dull- 
yellow brown. Cleavage angle, 89° 50’; specific gravity, 2 - 609. 
