404 Mr. Macgillivray’s Account of the Outer Hebrides. 
edges they pass insensibly into the surrounding parts, no lines of 
separation, however minute, being perceptible. They never pre- 
sent veins running out from them into the surrounding rock ; and 
when small they are more extended in a direction parallel to the 
seams of stratification than in any other. Fragments of these masses 
would of course pass for specimens of large-grained granite. 
Another source of variation is in the mineralogical nature of the 
ingredients. In the more ordinary kinds of gneiss, the mica is in 
scales, having a direction parallel to the seams,—the felspar in 
grains or concretions arranged in irregular lamin, and the quartz 
sometimes in similar grains, and sometimes in continuous plates. 
When the mica is more thrown into scaly lamine, a great change 
is produced in the appearance of the rock ; and so it may be re- 
marked of the other ingredients. One of these ingredients may be 
nearly or entirely wanting; and thus other varieties are produced. 
One of them may be substituted by another substance, as horn- 
blende or garnet ; or another ingredient may be added to the ori- 
ginal three. By far the most common of these additional ingre- 
dients is hornblende. In fact so common is it in the gneiss of these 
islands, that were gneiss to be defined according to its general con- 
stitution in this range of islands, the mineral in question would 
form one of its most essential constituents. Garnet is also a very 
common ingredient, and forms a predominant constituent of seve- 
ral varieties. 
_If to these sources of variation be added those presented by the 
changes of colour, to which the simple minerals entering into the 
constitution of the rocks are liable, some idea may be formed of the 
varieties which the gneiss of these islands assumes. The quartz, 
for example, may be white, bluish. grey, brownish, purplish, or 
dark brown; the felspar white or flesh-coloured. 
If felspar, quartz, and mica alone constitute gneiss, but a small 
portion of these islands belongs to that reck. On the other hand, 
the very common compounds of felspar, quartz, and hornblende, 
would require to be distinguished as a separate rock formation. 
The same is to be remarked of the gneisses having garnet as a pre- 
dominant ingredient. The circumstance of these varieties insen- 
sibly passing into each other, not merely by gradual interchange of 
ingredients in the direction of the planes of stratification, but also 
in different parts of the same stratum, is sufficient to justify their. 
being all referred to a common type. 
On the other hand, beds or strata often occur in which one or 
two ingredients only present themselves. Thus, a bed of what 
might be called mica-slate, occurs in the island of Pabbay, off the 
coast of North Uist,and numerous beds of characteristic hornblende- 
slate are to be seen in all the islands. These beds present them- 
selves in no regular order, and therefore, whatever the mineralogist 
may call them, by the geologist they ought te be considered as part 
of the gneiss formation. | 
The Sound of Harris contains four inhabited islands, Berneray, 
