marked it is very difficult to distinguish a hand-specimen of such gneiss from granite. This similarity is, of itself, sufficient to raise the suspicion that gneiss has been formed by the metamorphism and foliation of granitoid rocks.

Fine-grained gneiss has sometimes been formed from the crushing in situ of a coarser granitoid gneiss; compact gneissic schists which seemed to have been formed in this manner occur in Ross and Sutherland, and a slice from one of them near Loch Maree is represented in fig. 172.

With regard to the possibility of some gneiss and granite

Fig. 172. Fine-grained Granulitic Gneiss from Glen Laggan (Bonney).

having been produced by the extreme metamorphism and recrystallization of sedimentary rocks, Professor Bonney writes::- "It is often asserted that the process of metamorphism has been sometimes carried so far as to obliterate all trace of the original history of the rock; in other words, that rocks exist which, from their relations to other more intractable, and therefore less altered strata, appear to be of derivative origin, but which have become absolutely indistinguishable from members of the original or igneous group of rocks. This passing of a rock from the condition indicative of metamorphism to that characteristic of its having solidified from a state of fusion is, in itself, quite

conceivable, and undoubtedly there are cases where it is very hard to distinguish, in the present state of our knowledge, between an igneous and a metamorphic rock; but it is certainly a fact that the majority of instances brought forward in proof of this extreme metamorphism have broken down on careful examination." 1

From MS. notes by Professor Bonney, who adds, that "since this was written the advance of knowledge has shown that granite has, in many cases, been converted into gneiss, while no case of a granitic rock having been formed out of a mass of schist or gneiss has been established."

IN

CHAPTER XIII.

METAMORPHIC ROCKS.

Considered as Rock-Masses.

N the preceding chapter the metamorphic rocks have been described from a lithological or petrological point of view. We have now to regard them as rock-masses or components of the earth's crust; first describing the relations which their special structural planes (i.e., cleavage and foliation) bear to stratification and to one another, and discussing the evidence of the manner in which these structures have been produced; secondly, describing some actual cases and areas of metamorphism.

1. Cleavage Planes.

Dip and Strike.-The planes of cleavage may be said to have a dip and a strike just in the same way as planes of stratification. Their angle of dip varies from the vertical to as little as 10°, but is more frequently between 50° and 80°.

Fig. 173 illustrates the varying relations between the inclination of the strata and that of the cleavage planes. A represents a section near Llandovery in Wales, where a series of quartzose grits and sandstones are inclined at an angle of about 40°, and the cleavage planes dip in the same direction, but at a much greater angle. In в the cleavage planes dip in the same direction, while the strata are inclined in the opposite direction. In both cases the strike of the cleavage planes coincides generally with that of the

strata. Occasionally, when the strata have high dips, it happens that the cleavage planes very nearly coincide with those of the bedding and lamination.

Their strike is generally constant over considerable areas, and parallel to the direction of the axes of the principal flexures. This coincidence between the strike of the cleavage planes and that of the strata is of itself strong evidence that the cleavage was caused by the same pressure as that which caused the plication of the rocks.

One of the best examples of this steady direction in the strike of cleavage planes is the south of Ireland, over the whole of which, from Dublin to Mizen Head, the direction of the cleavage seldom varies much from E.N.E. and

a

Fig. 173. Relation of Cleavage to Bedding.

a a, Bedding planes. cc, Cleavage planes.

W.S.W., whatever rocks it traverses, and however different these rocks may be in lithological character and geological age. The strike of the main axes of flexure is likewise steady in the same direction. The planes of cleavage are inclined at very high angles, which generally approach the perpendicular, but when they have a dip, it is always to the S.S.E., and this southerly dip appears to be connected with the special form of the curves into which the strata have been bent. These are not mere undulations with vertical axes, but are pushed over in a northerly direction, so that their axes are oblique, and the strata are slightly inverted on the northern side, as if the pressure by which they were produced had acted most strongly

from the south; the dip of the cleavage is therefore roughly parallel to the dip of these oblique axes, both being due doubtless to the same cause.

It may, indeed, be stated as a general rule, that when the axes of the folds and flexures are inclined from the perpendicular, the average dip of the cleavage planes will be in the same direction.

North Wales is another good field for the study of cleavage. Fig. 174 is a section through the Snowdon chain, from the Menai Straits in a south-easterly direction; the beds, c c c, are conglomerates, the other beds being parallel to them, and the fine striæ are cleavage planes striking with the beds to the N.E., but cutting across them in the direction of the dip; for while the beds undulate at

B

Fig. 174. Cleavage in North Wales (after Jukes).

The fine lines indicate cleavage planes, but should have been drawn parallel to the arrows which show the slight variations in their dip. various angles, the cleavage dips slightly on either side of the vertical, viz., N.W. at 80° or 85° from A to B; S.E. at 80° to 85° from в to c; and 80° to the N.W. from c to D. From these facts it may be inferred that the rocks were folded first and cleaved afterwards, though possibly not long afterwards; and that some slight movements had taken place after the cleavage was set up.

In North Wales the prevailing strike of the cleavage is N.E. and S.W., because this is also the direction of the main lines of flexure; but where the strata are bent round so as to have a different strike, the strike of the cleavage is altered in a similar manner, so as to coincide with that of the beds.

"In the Berwyn chain, where the beds curve regularly round, from a N.E. and S.W. strike along the Bala and