PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 233 
innumerable parallel dispositions, stretched with perfect verticality* across the 
crystal, as the net of the fisherman is stretched by its leads with perfect verti- 
cality in a tranquil sea. But a consideration of what must be the relative 
motions of the molecules which are going to form a hemitrope crystal, would” 
necessitate our comparing their influence upon the white layers in such crystals, 
to that of a flood-tide impelling the buoy-rope and also the lead-line of a net the 
one way, while a medial current of ebb swept its corded structure in the opposite. 
In accordance with this, at the face of revolution of the two halves of the hemi- 
trope, the line of direction of this multitudinous net-work abruptly bends, and 
in its altered course maintains, as before, a perfect parallelism to the face a of 
the revolved segment of the crystal. (See figure 3, also Fare crystal.) 
But this white matter, if oligoclase, must not be regarded as merely extruded 
from the green, in the manner that amorphous impurities would be. 
As a definite chemical compound, it is itself subject to the operation of the 
crystallising force, in its double function as a physical separater and a symmetrical 
arranger. It is so arranged here, and the law of the arrangement is a singular 
one. It has been stated above, that the white reticulated structure is generally 
' parallel to the face a, and gives a glimmering cleavage reflection when re- 
volved about 4° either to or from the position of ordinary reflection of the face 
of the crystal asa whole. This glimmering reflection is unmistakeably from 
repeated cleavage faces, which give between each other an angle of about. 9°. 
Such a cleavage angle is unknown in felspar ; but the angle is as near that of 
the salient between the faces p of hemitropes of oligoclase, as reflections from 
so small surfaces could be expected to afford. Such hemitropes, however, are 
formed by the revolution of one-half of the crystal round an axis at right angles 
to the face m—that is, round the brachydiagonal—which, of course, is at 
right angles to the axis of revolution of the main crystal. 
In fine, we have here a squat crystal of orthoclase—squat as regards the 
length of the vertical axis—which crystal is a hemitrope accordant with face c, 
laced throughout by thread-like hemitropes of oligoclase (?), the main axes of 
the two being accordant, but the axes of the hemitropism being at right angles 
to each other. We have molecular repulsion coexistent with a peaceful crys- 
talline consorting in one and the same fabric—a consorting which, perhaps, is 
all the more firmly interwoven, that, as regards hemitropic arrangements, the 
substances uniting have agreed to differ. Perhaps only those who have studied 
the motions of the molecules which, in a fluent menstruum, are engaged in 
the building up of regular solids, can in any measure appreciate the intricate 
evolutions of the molecules of these two substances, as they jinkingly evaded 
one another in finding each its allotted place. 
* Supposing the crystal to be positioned as usual with the faces m vertical. 
VOL. XXVIII. PART I. 3P 
