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XX. On the Arrangement of the Foliation and Cleavage of the Rochs of the North of 
Scotland. By Daniel Sharpe, F.R.S., V.P.G.S. 
Received November 20, 1851, — Read February 19, 1852. 
Introduction. 
The mineralogical characters of the rocks to which this memoir relates have been 
so fully described by Macculloch and other writers, that it is not necessary to enter 
into any details on that head ; my object is to describe the usual arrangement of the 
layers of foliation which have given an appearance of stratification to Gneiss and 
Mica Schist, to show the relation of their foliation to the cleavage of the stratified 
slates, and to sketch out roughly the direction of the foliation and cleavage through 
the North of Scotland. 
The raosi; important remarks yet published upon these subjects will be found in 
the sixth chapter of Mr. Darwin’s “ Geological Observations on South America,” 
which, in addition to his own views and observations, contains a summary of what 
had then been done by others : Mr. Darwin’s remarks will be frequently quoted in 
this paper. I must also refer the reader to two papers on Slaty Cleavage, which I 
laid before the Geological Society in 1846 and 1848, and which are published in the 
third and fifth volumes of that Society’s Journal. 
The terms cleavage and foliation are here used nearly as they have been defined by 
Professor Sedgwick* and Mr. Darwin (p. 141) ; cleavage or lamination being applied 
to planes of division along which a rock may be split into thin parallel sheets inde- 
pendent of, and usually transverse to its stratification. Foliation describes the divi- 
sion of rocks believed to be of crystalline origin into layers of different mineral 
substances, whether the rock splits along those layers as in mica schist, forms a 
solid rock as in the granitic varieties of gneiss, or consists of distinct sheets of different 
materials, such as quartz, mica, chlorite, &c., which sheets may be of any thickness, 
from a line up to 1 or 2 feet, as maybe seen both in the “laminar gneiss” and 
“ chlorite schist series ” of Macculloch. For the sake of clearness, the term slate is 
here confined to laminated rocks with a cleavage independent of their stratification, 
and schist to foliated rocks with only one set of divisional surfaces produced by the 
foliation. The distinctions between foliation and stratification, which Macculloch 
and many subsequent authors have confounded, will be easily understood when the 
various peculiarities of foliation have been described. 
Under whatever aspect we regard them, it is difficult to draw any clear line of 
* Transactions of the Geological Society, Second Series, vol. iii. p. 480. 
