PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 229 
So that in this crystal the amount of the foreign matter now represented by 
: ; 1 
pores is about one-eleventh and a third, T-3° of the whole crystal. A remark- 
able coincidence with the proportions in the Arran crystal. 
I find that many crystals of orthoclase exhibit these vacuities in their sub- 
stance, exactly corresponding in position to that of the structure which has been 
described. Crystals of adularia from St Gothard are frequently hatched, and 
cross-hatched in twins, through the lineation produced by this deficiency of sub- 
stance. I possess a crystal which is little better than a skeleton from this cause. 
4. What is the Material of which this knitted Structure is composed ? 
It would not, as before mentioned, be practicable to separate the material 
from the orthoclastic mass: by a consideration, however, of the composition of 
those felspars which exhibit the structure, a conclusion, which it would not be 
unreasonable to consider as closely approximative to the truth, may be drawn. 
These analyses show so slight a departure from the average analyses of 
orthoclase, that it is evident that it cannot be a substance very far removed 
in composition. The only decidedly noticeable differences are an excess of soda 
over that normal to orthoclase, and the introduction of some lime. 
The first point here to be noted is that as the amount of silica is quite normal, 
and as the lineation is the softer part of the structure of the crystal; it cannot con- 
sist of quartz: had it done so its occurrence in the position occupied would not 
have been altogether inexplicable, for then it would have consisted of the excess 
of silica over and above that necessary to saturate the bases in the formation of 
felspar ; which excess indeed the quartz in granite veins must probably all be 
regarded as being. Such an excess of silica, and so separating, forms in the 
graphic felspar of Ben Capval and Stromay, a structure wondrously similar to 
that under consideration. 
The sonnenstein appearance of the layers gives rise at once to a suspicion of 
their being composed of some species of felspar ; but how a felspathic material 
should, in thus being extruded from another of similar nature, form so strange 
a structure, instead of regular crystals, is the problem to be solved. 
Now, seeing that other felspars are paragenetic with orthoclase in veins, 
that these are softer than it, and, all but albite, more readily decomposed, it is 
very probable that the material consists of one or other of these. 
There are, however, for lithological relationships afterwards to be pointed out, 
only two that it can well be—albite and oligoclase—and the evidence appears 
to go entirely in favour of its being the latter.* 

* Were it not so, we would have to regard these as being all microlines ; but DESCLOISEAUX, 
| (Comptes rendus, April 17, 1876,) holds these to consist of a mixture of orthoclase, microlin, and 
VOL, XXVILI. PART I. 30 
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