CLEAVAGE OF THE ROCKS OF THE NORTH OF SCOTLAND. 
459 
any propriety in retaining the name which implies it. The materials of gneiss and 
mica schist may have previously existed in some other form ; but that alone is not 
enough to induce us to call them metamorphic; for the name might be applied for 
the same reason to granite and syenite, or even to lava, all of which have probably 
undergone various mutations. It would be better to restrict the term metamorphic 
to rocks of a sedimentary origin which have undergone changes affecting only their 
mineralogical character without losing the evidence of their bedding. It will also 
add to the clearness of our language if we confine the term slate to laminated rocks 
of stratified origin, and call the fissile foliated rocks schists; in which manner the two 
words have been used in this paper. 
Appendix . — Among the attempts made to explain the cause of the lamination of 
slaty rocks, reference has often been made to the interesting experiments of Mr. R. 
W. Fox, since repeated by Mr. Hunt, upon the changes produced by long-continued 
voltaic electricity upon masses of clay, bricks, &c.*. The results produced have a 
sufficient analogy, both to lamination and foliation, to rouse our attention to the 
subject, without coming near enough to the great features described in this paper to 
allow us to regard them as a solution of the problem. In the mass of clay operated 
on by Mr. Hunt (fig. 12, p. 451), one side has assumed a laminated and the other a 
foliated structure, the latter being curved and more contorted than the former, as is 
usually the case in similar structures in the earth ; but we do not see the regularity 
which is so striking in laminated rocks, and the two structures, instead of harmonizing 
as in nature, appear in a sort of opposition, being produced on opposite sides of the 
mass in an unconformable position. 
The changes produced in the brick (51) and plaster of Paris (49), p. 453, are more 
analogous to what may sometimes be seen in rocks traversed by fissures or mineral 
veins than to slaty cleavage. In these experiments the mass acted upon was found 
to be laminated on the side nearest the zinc plate of the battery, and considerably in- 
durated on the copper side. 
In examining the coast of Portugal near Cascaes, I met with several parallel per- 
pendicular fissures between 10 and 30 feet wide, which run N.W. from the coast 
about a mile west of Cascaes towards the granite hills of Cintra, through beds of 
limestone and sandstone of the ages of our lower greensand. Where these fissures 
cut through the cliff they may be well examined, and the following notes were made 
respecting one of them. The cliff consists of thick beds of sandstone, in which all 
the minor traces of the bedding are obliterated for a breadth of 2 or 3 feet on each 
side of the fissure, only the great divisions remaining visible, and for about 6 inches 
on each side of the fissure the sandstone is laminated, splitting in perpendicular layers 
parallel to the walls of the fissure. The crack itself is filled by vertical layers of sandy 
* Memoirs of the Geological Survey, vol. i. p. 451 and 453. 
