26 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
accordant with the micaceous lamination of the rock, following obediently that 
lamination where it has been crumpled * 
This approaches, though it does not come up to,—it resembles, though it is 
really different from what is to be seen at Clach-an-Eoin. 
The study of the Boggierow rock leaves the impression that the felspathic 
portion had attempted to arrange itself as crystals, or had been crystals, 
porphyritically disposed. Such a conclusion will hardly apply to the Innisbae 
rock ; the felspathic matter is certainly not porphyritically disposed when it is 
confined to a regular arrangement in layers; and such a conclusion certainly 
will not apply at all to what obtains at the northern locality. First, it will not 
apply in s¢ze; the individual collections of felspathic matter, to which inches 
applied at the other localities, are here of the dimensions of feet and yards. 
Second, it will not apply as to internal structure; a certain amount of rough 
cleavage which is to be obtained in the first cases showed that each—all being 
much of a size—was to be regarded as an individual mass, of which the 
components were zs molecules; here nothing like cleavage is to be got; the 
components are crystals, granules, plates, promiscuously agglutinated, and 
forming masses of greatly varying size. ‘Thirdly, it cannot apply as to shape ; 
there is here no trace of geometric form, for the masses are lenticular. 
The strike of the rock is north by east and south by west; it stands nearly 
vertical ; its bedding is well shown by the parallel. disposition of its layers 
of black mica; it is singularly free from all plication; but, between the 
bedding of its mica sheets, there occur in marked abundance, but at quite 
irregular distances, parallel arrangements of the segregated felspar of the 
rock, disposed like the glands on a duct, or ganglia on a nerve, the enlarge- 
ments being of ever-varying size. 
In one respect the comparison with ganglia on a nerve is not satisfactory; the 
felspathic bands are generally not continuous, but the juxtaposition of the two 
micaceous layers which Jately sheathed what I have represented under the 
figure of ganglionic enlargements, leads, in straight course, to the next, and 
not far separated lenticular mass. 
There can be little room for doubt that this is a modified development of 
that segregatory process in virtue of which the felspathic material of gneissose 
vocks so frequently arranges itself in layers or bands. As these bands consist of 
a material more plastic than the less fusible quartz and mica, they are, in the 
ordinary case, when plicated, thinned off to nothing at the more compressed 
flexures, only to re-appear in ampler development among the loosened or more 
drooping folds. But at Clach-an-Eoin we have no plications to compress or 
* Something very close to this is given by Corta as his description of typical porphyritic gneiss—“ In the 
otherwise uniform schistose mass there occur at intervals large egg-shaped crystals of orthoclase (sometimes 
amorphous), round which the foliated texture bends itself with a wavy sweep.” Ruskin, in treating of the rock- 
structure of the Alps, gives an admirable drawing of such gneiss. 
