CHAPTER XII.

PETROLOGY OF METAMORPHIC ROCKS.

§ 1. Structure of Metamorphic Rocks.

METAMORPHISM has already been defined (p. 255),

and the various rocks which are included in the Metamorphic series have been enumerated and described so far as concerns their lithological characters and constitution. In the present chapter we have to describe the conditions under which these rocks occur, but before entering upon this subject we must give some account of the structures which specially appertain to metamorphic Derivative rocks, and which often supersede the laminated or bedded structure that they originally possessed. These are the structures known as Cleavage and Foliation.

Cleavage. A general definition of cleavage and a description of the lithological characters of slate have already been given (pp. 294 and 296); but there are many other facts connected with cleavage which come more properly within the scope of the present chapter. It has been stated that cleavage is a structure produced by intense compression, and the following statements may be regarded as so many proofs of this assertion.

Arrangement of Particles in Cleaved Rocks.-By microscopical examination Mr. Sorby found that the minute particles of slate lie more flatly in the cleavage planes than in any other direction; whence he inferred that they had been lengthened in the direction of the cleavage planes, or else, if they were originally of unequal dimensions, they have been re-arranged so that the longer axes coincide with the planes of cleavage. In the case of coarse-grained rocks this re-arrangement of the component particles may be easily observed; thus, Sir A. Ramsay describes a con

glomerate near Llyn Padarn in Caernarvonshire as consisting of slaty pebbles in a slaty matrix, the whole being affected by cleavage remarkable on account of the pebbles being elongated in the direction of the cleavage planes, and obliquely to the bedding."

Mr. Sorby also noticed that the green spots so often visible in slates were always elongated along the cleavage planes, and compressed in a direction at right angles to them, while elsewhere the similar spots were spherical. Since, however, these spots have probably been caused by the decomposition of organic matter, it is very doubtful how far this observation is of value.

The same appearances are exhibited by the fossils which occur in slate rocks; their dimensions are always changed, so that the fossil is distorted, being lengthened in one

B

Fig. 141.

direction, and shortened in that at right angles to the cleavage planes. Thin shells such as Lingule and the tests of Trilobites are especially affected, the latter being sometimes minutely wrinkled in lines parallel to the cleavage. If we suppose a force acting from the direction c, upon the body æ, in fig. 141, this may be resolved into forces acting from A and B, one tending to flatten and broaden it, the other to roll it out; Prof. Haughton has shown that if the fossil originally lay flat in the plane of bedding, the greatest distortion or elongation is in a direction parallel to the intersection of the cleavage and bedding planes.

Mr. Sorby has also shown that when harder beds, such as bands of sandstone or limestone, occur among slates, they are strongly contorted in the manner represented in fig. 142, their dimensions being contracted at the sides,

Geology of North Wales, Mem. Geol. Survey," p. 145.

and expanded at the tops and bottoms of the curves, while the beds of slate above the sandstone are little, if at all bent. He infers that the particles of the slates must have been compressed at right angles to the cleavage planes, and lengthened along them so as to allow of their being squeezed into the same contracted space as the sandstones, without much bending of the surfaces of the beds. Measurements along some of these contorted bands show that the whole mass has been compressed into one-third of the linear or horizontal space which it originally occupied. Mr. Sorby remarks "that these and numerous other cases in North Devon are analogous to what would occur if a

strip of paper were included in a mass of some soft, plastic material which would readily change its dimensions. If the whole were then compressed in the direction of the length of the strip of paper, it would be bent and puckered up into contortions, while the plastic material would change its dimensions without undergoing such contortions; and the difference in distance of the ends of the paper, as measured in a direct line, or along it, would indicate the change in the plastic material."

Cleavage is always most perfect in the finest grained rocks, splitting them into an indefinite number of thin plates. The coarser the rock, the fainter, the wider apart, and the more rough and irregular do the cleavage planes

become; and in the case of thick-bedded sandstones cleavage generally divides the rock into coarse slabs only, the upper and under surfaces of the beds often breaking into dog-toothed indentations, and thus indicating the compression they have undergone.

Dip and Strike of Cleavage Planes. 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°. 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.

Fig. 143 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 B 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.

In some places the cleavage planes coincide in dip and direction with those of the bedding and lamination, as is

FF

the case with the slates and schists at Abereiddy Bay, Pembrokeshire (fig. 144).

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 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

Fig. 144. Cleavage coinciding with bedding, Abereiddy Bay.

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 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.

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.