PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



wavy lines, which at intervals are crossed by roughly parallel,, 

 thicker, dark bands, so as to present a good imitation of false-bedding ; 

 but the latter are quite absent from some parts of the block (fig. 4). 

 On close examination of specimens, we find every transition from the 

 simpler to the more complicated structure. These broader bands 

 are either records of the strain-slip cleavage or of that strained 

 compression of layers (which may eventuate in cleavage) formed in 

 a series of overfolds. As an extreme case, these darker, or secondary 

 layers, as we may call them, dominate over others. The rock, then, 



Quartzose gneiss (chiefly quartz and biotite). Stratification-foliation (a) modi- 

 fied by pressure approximately at right angles to it. The mica-flakes in 

 the bands («) are mostly parallel with line b. About natural size. From 

 the cliffs at Muchalls (Aberdeenshire). 



when acted upon by the weather, tends to split along them, and the 

 surfaces are seen to exhibit the characteristic sheen already men- 

 tioned. The most perfect examples of this can be seen on the rocky 

 shore, a short distance south of Muchalls, close to a target. 



On examining specimens of these rocks microscopically, we find 

 that one of those in which a wavy banding only is exhibited, chiefly 

 differs from an ordinary rather minutely granular quartzose gneiss 

 in a general parallelism of the flakes of dark mica to a plane roughly 

 at right angles to the average direction of the planes of foliation(fig. 3). 



Fig. 4. 



Quartzose gneiss (as in fig. 3). Part of a block, showing the original stratification - 

 foliation (a) modified by pressure and producing a cleavage-foliation {b) by 

 strain and rupture of flexures in part of the specimen, thus mimicking false- 

 bedding. The drawing is diagrammatic, about ^ natural size. From a 

 fallen block smoothed by waves just south of Muchalls. 



Throughout the rock also the constituents, if irregular in form, 

 seem to have a similar orientation. The rock, in its present con- 

 dition, does not exhibit marked indications of crushing. When we 

 examine specimens of the apparently false-bedded rock, we find 

 that the quartz and the felspathic granules have a more fragmental 



