446 
MR. D. SHARPE ON THE ARRANGEMENT OF THE FOLIATION AND 
distinction between gneiss and mica schist; considered mineralogically, they are the 
two extremes of a series of foliated crystalline rocks^ between which every shade of 
variation may be found ; their geological relations are the same, and the foliation of 
the two rocks follows exactly the same laws. Their geographical separation through 
Scotland has been drawn most arbitrarily ; the lines which divide them on Maccul- 
loch’s map have very little meaning, and there are many large tracts where the 
colouring might be reversed without any impropriety. The same may be said of the 
chlorite schist series of Macculloch, which, where I have seen it, is a variety belong- 
ing to the same class of rocks, but more nearly related to gneiss than to schist, in 
which the mineral ingredients are more numerous, and more completely separated 
from each other in parallel layers than is usual in gneiss or mica schist ; yet there 
are many districts in which the separation of the ingredients of gneiss into layers is 
as complete as in the chlorite schist described by Macculloch^, and in which the 
quartz forms layers between one and two feet thick, alternating with layers of mica- 
ceous schist, or with beds of a crystalline rock without foliation or cleavage. The 
other varieties of schist described by Macculloch belong to the same series of rocks, 
and agree with gneiss or mica schist in everything but mineral composition. The 
difference of their constituent parts appears to have produced no alteration in the 
foliation of these rocks, which follows the same laws in them all ; the only variation 
in that respect consists in the greater or less completeness of the separation of the 
different minerals into parallel layers ; and these are but different stages of one pro- 
cess. For the sake of brevity, therefore, I shall use the terms gneiss and mica schist 
to include all the varieties of foliated rocks. 
Under the name of quartz roch, Macculloch has classed in connection with gneiss 
and the various schists, two sets of rocks belonging geologically to two most distinct 
classes, and only resembling each other mineralogically. 
The quartz rochs of Glenorchy and Tyndrum^j-, with most of the varieties composed 
of quartz and felspar, or quartz and mica, enumerated in Macculloch’s classifica- 
tion of rocks, belong to gneiss, from which they differ only in simplicity of composi- 
tion, and with which they are usually found ; these are foliated rocks, and are here 
regarded as so completely part of the gneiss as to require no separate mention. But 
the greater part of the quartz rock which figures on the map of Scotland is an 
altered sandstone, of which the mineral character has been changed by platonic 
action ; it is a sedimentary deposit in which the stratification is usually distinctly 
visible. The difference between these two quartz rocks is so obvious, that they may 
be distinguished even in reading Macculloch’s descriptions. The latter forms no 
part of the subject of this paper, being a stratified, and not a foliated rock; but it 
W'as necessary to mention it, lest any of the following remarks on the subject of 
foliation should have been supposed to apply to it:|:. 
* Western Isles, vol. ii. p. 284. f Macculloch, Transactions of the Geological Society, vol. ii. p. 478. 
t I have entered more fully into this subject in a paper laid before the Geological Society. 
