PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 245 
On 25 grains— . 
Silica, . , : erg de WOLDS 
Alumina, . ; : an ont 
Ferric Oxide, nas 1) al 249 
Magnesia, . , ; *32 
(ii Ae Ne abe Mie ery?) 
Potash, : ‘ : Bei kat 5y4 
Soda, . cats Se : oe) ee 
Water, i) 2 : ; ‘ ‘54 
99 - 666 
Insoluble silica, : 922 per cent.; possible impurity, orthoclase. 
-_ From the Vein in the Syenitic Granite near Lairg, Sutherland. 
8. Occurs along with the orthoclase,—the analysis of which has been - 
given,—and the associated minerals already mentioned. 
The oligoclase is in crystals of about two inches in size, fapedden in the 
paste of quartz. These crystals are yellowish cream colour, to white ; opaque ; 
dull throughout ; and not striated, but cleaving so readily at the twin ae as to 
appear somewhat foliated. 
In breaking up a large crystal, a portion was obtained from the centre 
which was colourless, translucent, and distinctly striated. The non-striated 
_erystal gave the angle 86° 10’; the striated portion, the angle ‘86° 15’. The 
specific gravity of that portion was 2 - 618. 

a Cream-coloured. Colourless. 
Silica, . Oe LS. y eo? 1052 62-813 
Alumina, ae ce 22,919 
Ferric Oxide, - . op oer - 156 
Macnesiayy |... ye) “14 08 
Lime, . ; : eee 4°25 
Potash, . ; ‘ : wes - 84 
Soda, °- : ae a is 8653 
Beiatersies gts dt Be 28 “29 
99 - 878 
_ Possible impurity of first, orthoclase. 
The striking fact here is, that what may be regarded as incipient weathering 
_ obliterates absolutely the reflection which developes striation. 

From bedded Porphyry. 
9. ‘The mode of occurrence of the oligoclase found in the porphyry of 
|Canisp has been noted before. Occasionally a small crystal of the red ortho- 
VOL. XXVIII. PART I. es 38 
