PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 9 
The protrusion of granite veins above the ordinary level of the rock—quite 
a familiar fact—may be partly due to their less minutely granular, that is, less 
uniformly porous and loose-grained structure ; but the almost invariable pro- 
trusion of the plates of mica, even above the quartz, in granite veins, vouches 
unmistakably for the greater endurance which that mica must impart. 
The not unfrequent occurrence of loose blocks of large-grained granite 
in straight line across a heath-covered or grass-clad surface points also to 
a former vein, which had longer resisted the denudation due to atmospheric 
decay than had the inclosing rock. 
This is a subject which, as a whole, has certainly not engaged the amount 
of attention which it merits. Probably the following structures have to be dis- 
criminated, and have been more or less confounded with one another :— 
Ingredients same as those of the Rock-Mass itself. 
1. Plugging pre-existent Rents. 
Contemporancous plugs of rents in igneous rocks 
intersect rock in curving and angular manner; structure smadller than that of the rock- 
mass; both branching and intersecting; formed by sudden injection. 
Veins of eafiltration 
intersect rock in curving and angular manner; structure Jarger than that of con- 
taining rock; both branching and mutually intersecting; formed by a single con- 
tinuous process. 
Metalliferous veins 
frequently intersect more than one rock-mass; generally rectilinear but angular ; struc- 
ture larger than that of containing rock; branch,and cut rocks of different natures ; formed 
by intermittent actions, diverse in their natures, and markedly so in their products. 
2. Not filling pre-existent Rents. 
Bands of dominant crystalline action 
accordant with the floor or surface of igneous flow ; structure larger than that of contain- 
ing rock; neither branching nor intersecting ; frequently spherulo-radiate in structure. 
Bands of metamorphic seggregation 
accordant with flexures of rock strata; structure larger than that of containing rock ; 
neither branching nor intersecting ; of ever-varying thickness in plicated rocks. 
Veins of seggregation 
angular, and intersecting rock strata; structure larger or smaller than containing rock; 
both branching and intersecting ; generally accuminated at their terminations. 
Ingredients not the same as those of the Rock-Mass 
intersect more strata than one, in all directions and ways; frequently branch and 
intersect each other. 
a. Granitic dykes, structure larger than including rock ; frequently fill faults. 
b. Trap dykes, structure smaller than including rock; fill rents. 
VOL, XXIX. PART I. Cc 
