PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 5) 
Second,—The occurrence in the veins of fractured, and occasionally of 
apparently floating angular fragments of the rock, likewise points to an actual 
solidification of the rock previous to the disruption and envelopment of the 
fragments. 
Third,—The gradual increase in the size of the crystals which fill these 
veins as they approach the centres thereof, the frequent capping of the quartz 
crystals (a recognised proof of intermittent growth), and the fact that any free 
crystalline summits which may be present invariably point to these centres, 
show that a sudden injection from without, and consequent more or less uni- 
form and rapid cooling and solidification, could not be the manner in which 
the rents were filled. The relationship of the vein to the rock mass itself also 
negatives injection. There is no dine of separation between the vein and the 
rock ; the granular structure of the rock is, in narrow space it is true, but 
still gradually augmented in size; and this ampler crystalline structure of the 
vein seems to grow out of, to be rooted in the substance of the rock. 
The change of structure takes place within the space of an inch, but within 
no part of that inch can the stroke of a hammer effect a separation. 
The information conveyed by the hammer is indeed of a most instructive 
description. : 
Granular though the structure of granite is, its grains do not lie in con- 
fused arrangement. It may not be to the smallest extent bedded in the 
quarry, though it generally is; but granite has a perfect cleavage and cross 
cleavage, so determinate in their directions that the workmen speak of the 
bedding of even such quarries as present great faces of apparently perfectly 
continuous and unvarying rock. 
These cleavages result from a general polarity in the crystals of felspar, 
which have their axes, and hence their cleavages, lying in the main in one 
direction. Thus the quarryman by blow and cross blow cuts the rough 
paving stone, so as to leave himself only the narrower faces to “dress to 
square.” 
But, inasmuch as the crystals of felspar which grow out of the rock to fill 
these exfiltrative veins do so at right angles to the sides of the vein, and as no 
one of the cleavages of felspar is at right angles to its main axis, the hammer 
blow cannot effect a separation throughout any part of the space wherein this 
rectangular riveting of the two structures is effected. 
It would be far from easy to adduce any evidence which could more 
conclusively show that the present contents of these veins had exuded from 
the rock mass itself; and it need hardly be noticed in this connection that 
these veins never contain a single mineral substance which is not to be found in 
the rock mass itself; though, in the latter, many of these substances are of 
difficult recognition from their comparatively very small magnitude. 
There is a fourth feature, moreover, which is not infrequent, namely, that 
VOL. XXIX. PART T. B 
