1-KOM THE CROSS FELL INLIEK. 513 



divisional surfaces thus obtained are seen minute wrinkles havinf^ 

 the same direction as the broader undulations, while a cross-section 

 shows that these small folds have for the most part passed into 

 little parallel faults making a high angle with the bent surfaces 

 of lamination. In a thin section | JJKij * the gradual passage of 

 the minute folds into reversed faults is l)eauti fully exhibited, and 

 many more are brought to light than can be detected in a hand- 

 specimen. There are sometimes as many as two or three hundred 

 in an inch, or even more. All the appearances recall the micro- 

 structure of the " gnarled " beds near Amlwch, &c., in Anglesey f. 

 In the specimens from Erownber the wrinkled lamination-surfaces 

 present a dark and glossy aspect, which the microscope shows to be 

 due to the development of a chloritic or micaceous mineral in the 

 rock. 



The chief secondary product is a flaky mineral showing the 

 strong cleavage of the micas, chlorites, &c,, and giving sensibly 

 straight extinction. The flakes vary from pale greenish-yellow to 

 colourless, the absorption being stronger for vibrations parallel to 

 the cleavage-traces than for those perpendicular. The least axis 

 of the ellipsoid of optic elasticity is at right angles to the cleavage. 

 The birefringence, roughly estimated by comparison of the polariza- 

 tion-tints with those of quartz, is usually about 0-012, but some- 

 times as much as 0-014. These figures correspond in the table of 

 Levy and Lacroix to clintonite and delessite respectively. Pur t her, 

 there are in places small colourless flakes giving much higher 

 double refraction and agreeing in character with muscovite. It is 

 evident that, besides the dominant chloritic mineral, a micaceous 

 one is also present, and the appearances suggest that the latter 

 represents a further stage of metamorphism than the former. The 

 brilliantly polarizing mica appears only on planes of actual discon- 

 tinuous movement in the slate or in little isolated flakes in the 

 gritty bands, and these are evidently the places where the mechan- 

 ical stresses developed would reach a maximum. It appears that 

 the discontinuous movement in the mass of the rock has been 

 efiected after the production of the chloritic mineral which almost 

 completely pseudomorphs the original argillaceous material, and the 

 flakes, except where they have been dragged along in the slipping, 

 lie obliquely to the little faults. 



The gritty bands in the rock sometimes retain their clastic 

 appearance, but in some cases their appearance suggests recrystal- 

 lization in siH(. The constituents are quartz and felspar, among 

 which occur sparsely flakes of the chloritic mineral and the colour- 

 less mica. The quartz often shows something of the " undulose "' 

 or " spectral " poJarization indicative of a condition of strain. The 

 felspar is frequently twinned, and seems to embrace both orthoclase 

 and an acid plagioclase. The perfectly pellucid character of the 

 little crystal-grains and, in some places, the fashion in which they 



* The numbers in square brackets refer to the microscopic rock-sections in 

 t!ie collections of the Woodwardian Mu?eum, Cambridge. 

 t See Kep. Brit. Assoc, for 188r>, pp. 8o9, 8-IU. 



