PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 231 
5. Has this included Foreign Matter any effect, and what, upon the 
enclosing Crsytals ¢ 
The cleavage angle of every one of the felspars showing this structure 
departed more or less from 90°; and the departure from a right angle is greatest 
in the felspars which show the structure in tts strongest development. 
Lairg, 89° 39’; Rubislaw, 89° 58’; Stromay and Ben Capval, 89° 50’; 
Tongue, 89° 43’; Cowhythe Head, 89° 40’; Blirydrine, 89° 40’; Eslie, 89° 40’ ; 
Anguston, 89° 49’; Yestnaby, 89° 56’, are a few quotations,—these being, out 
of measurements of several fragments, those which were the most common ; 
though occasionally measurements nearer to 90° were obtained, none were 
absolutely at the right angle. 
If we suppose that the foreign substance is oligoclase, and that the polar 
axes lie accordant with one another,—which it will be shown that they do,—the 
obliquity of the cleavage angle in that substance should distort more or less the 
orthoclastic cleavage. 
The angle of oligoclase differs 230’ from that of orthoclase ; the amount. of 
oligoclase present seems about yy ; the eleventh part of 280’ is 21’. 
The case of those crystals which, like the Murchisonite, are systematically 
porous, here calls for consideration. The crystal from Arran, being a complex 
twin, was unfitted for the determination of the angle. 
If the open structure and the filled-up structure be in reality the same, and 
if the crystals now vacuous be also more or less triclinic, we must conclude that 
the oligoclastic material segregated out of the orthoclastic when both were 
solidifying, nearly contemporaneously ; and that in the hollow crystals the more 
~ decomposable mineral has been weathered or dissolved out. All Murchisonites I 
have seen were either loose stream-rolled crystals, or they were weathered; but it 
was not sowith the Hillof Fare specimen; itwas taken froma freshly-opened cavity. 
It cannot be supposed that the crystalline molecules of the orthoclase 
arranged themselves preconcertedly, so as to distort the crystal ; and also so as 
to leave vacuities for the reception of a pre-selected substance. The action of the 
erystallipolar force, in its wondrous production of hemitrope, twin, hemihedral, 
and hemimorphic crystals, must be said to be determinanve, but we cannot assign 
toit the function of being deliberative. And though it is quite conceivable that 
pre-existent vacuities might be plugged up by the accretion of a subsequently 
solidifying felspathic material, or, in its absence, of the siliceous paste, still it is 
inconceivable that vacuities—the absence of material—could distort the angle 
ofacrystal. “Nothing can come of nothing,’—nothing can do nothing. 
An insufficiency of accreting molecules produces modifications—new faces on 
a crystal, but is powerless to alter its angles ; and we are thus forced to con- 
clude, as above, that the contemporaneous, or nearly contemporaneous, solidifi- 
