226 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
quently appear opaque, white, or brownish red,—always lighter in colour than 
the general mass of the crystal; but this opacity is, except in the case of 
weathered crystals, the result of the manner in which the light is reflected from 
them. 
Placing the crystal as usual, with the macrodiagonal transverse to the 
observer, and reflecting light from the general surface of the face ¢, the striz 
pass as dull or opaque white interrupted lines from the observer, the face itself 
frequently exhibiting a faint lineation between the fibres, parallel to the 
macrodiagonal ; but on revolving the crystal upon the diagonal about 4° either 
- to or from the observer, the corded structure starts out with anacreous glimmer, 
somewhat similar to that in sonnenstein; the edge between the two reflecting 
faces lies parallel to the macrodiagonal of the large crystal, and the light 
appears to the lens to be reflected from innumerable interrupted cleavages. 
When the somewhat imperfect a cleavage is obtained, a less interrupted and 
somewhat more brilliant flash is thrown from spots upon its surface ;—careful 
observation being required before the impression is done away with that the 
light is reflected from thin plates of tale. 
In the more brilliantly green-tinted and transparent amazonstones, the 
colour of these so-called fibres is usually of an opaque white; in some duller 
crystals they are of exactly the tint of dead muscle. 
2. Is the Material of the duller portions diferent from that of the 
general Mass? 
In no specimen that I have seen is it within the bounds of possibility to 
separate the one from the other, so as to determine this by actual analysis ; but 
it may reasonably be concluded that they are different from (1st) the higher 
lustre of the filamentous portions when unweathered ; (2nd) the greater readi- 
nesss with which these portions do weather ; (3rd) their greater opacity when 
weathered ; (4th) their inferior hardness,—a soft knife drawn across both does 
not scratch the general mass, but only the fibrous portions. 
3. What proportion of the Crystal does this intruded or extruded material 
bear to the ordinary Orthoclastic substance ? 
The habit of taking the specific gravity of all minerals examined led to my 
being able to form a fairly approximate estimate of this. 
Many determinations of specific gravities. of orthoclase from diverse locali- 
ties give an average of 2°555. 
A crystal. of the variety called Murchisonite, obtained at Loch Ransa, in 

