PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 225 
When viewed only on the surface of an opaque crystal, the structure 
resembles, in its somewhat interrupted markings, a graphic delineation of the 
distant waves of the sea; or the light reflected from a polished piece of fine- 
erained ivory, cut parallel to the length of the tusk. When viewed in a 
transparent crystal through a lens, it resembles the fibres, great and small, of a 
partially untwisted cord, spread loosely and more or less uniformly across the 
crystal—the direction being invariably parallel to the face a of Brooke and 
MILLER (ii. of Dana). 
Professor Nicot of Aberdeen, who quite independently observed the same 
thing in the crystals of amazonstone lately obtained in Sutherland, writes me 
(12th Nov. 1875) as follows :—‘‘ The most curious felspar I have seen is one 
from a boulder near Ribigill. It is a macle like those im p. iii. of my ‘Elements,’ 
but broken at the ends; it is about 3 inches long by 2 broad. It consists of red 
and green layers running across the cleavage face, and down the side of M, 
approximately parallel to its edge, and at right angles to the plane of cleavage, 
almost like fibres of wood or muscle. The specimen, I understood, was from a 
druse in a boulder broken up when clearing some fields. The brown and green 
are not in twins, but mixed in irregular plates—the cleavage runs right across 
both. The layers look like fat and flesh fibres in a piece of well-mixed beef. 
These fibres are not quite continuous, but more or less interrupted or broken. 
They are thicker near the outside. 
“ You call it amazonstone, but I think it is rather orthoclase, mixed with 
the green mineral. This is more gem-like and less altered than the red. I am 
‘inclined to think also harder.” 
Up to the time of receiving Professor Nicot’s letter I was uncertain whether 
these markings were to be regarded as due to a peculiarity of structure, or to an 
actual difference in material ; though I inclined to the latter belief from having 
| observed the fact, noted above by Professor Nico, that the fibrous-like sub- 
| stance invariably weathered faster than the general mass of the crystal. The 
Opinion of so acute an observer as Professor Nicon strengthened me in this 
_ belief; but it was not until I had analysed many such felspars, and more 
minutely examined the structure, that I definitely came to the conclusion that 
| Nicot’s view as to their being actually two chemically different substances con- 
joined in one crystal was the correct one. 
The observations I have made, so far as yet carried out, are the following :— 

1. The Structural Appearances. 
These fibres and fibrille invariably differ in colour, lustre, and transparency 
from the general substance of the crystal through which they are spread. When 
viewed perpendicularly to the face c,—that of most perfect cleavage,—they fre- 
PART XXVIII. PART I 3N 
