
PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 271 
for testing their gravity, after having examined with the lens their myriad 
chips in purifying them for analysis, and after having analysed them,—that is, 
after having obtained that absolute knowledge which enabled me to correct my 
own errors,—I should not even now like to have to pronounce upon certain 
obscure varieties ;—but, that if I did see such an amount of recognisable 
features as to warrant the expression of an opinion, I should be inclined to 
maintain it against that of any one whose eye had not been educated to a 
similar extent. 

The microscopic structure and optical properties of the felspars, with the 
aid to their recognition thereby afforded, will be considered in Part II. of this 
paper. 
Andesine, anorthite, and Latrobite are now first introduced as British 
minerals. : 

EXPLANATION OF PLATES XVII. AND XVIII. 
Prats XVII.—In the centre is figured to size one of the crystals of amazonstone from Tongue. The 
figure on the left side thereof shows the appearance, with polarised light, of a slice 
of the same, cut parallel to c; magnified about 25 diameters: the undulating 
“corded structure” runs parallel to a, and exhibits gradations of colour on account 
of a difference in the thickness of the slice at its two edges: the twinned 
and dovetailed lineated structure, which is at right angles to the other, is parallel 
to b. The figure on the right shows the appearance of the “corded” structure in a 
slice cut parallel to d. 
Prats XVIII.—Some of the forms of the Tongue crystals. No. 1 is in the possession of Professor 
Nicon of Aberdeen. Nos. 6 and 7 (Murchisonites from Arran) belong to Mr 
Duperon of Cargen. The others are in the collections of Mr Dupczon and the 
Author. 
