ON CLEAVAGE AND FOLIATION IN ROCKS. 



383 



Fig. 20. cleavage and foliation. It can, however, scarcely be 



doubted that in this district bands of nearly vertical 

 cleavage alternate with bands of cleavage inclined 

 40°, 50°, 60°, 70°. The reference of thise dips to 

 ^ certain anticlinal and synclinal* axes is the part of 

 Mr. feharpe s view which specially requires the atten- 

 s ^'''t observers both in Scotland and in Switzerland. 

 ^^ Mr. Sharpe obtained results of the same general 



- character in the Highlands, but with the vertical 

 bands (synclinal axes) much further apart than in 

 ttie Alps, and the anticlinal arches very much flat- 

 tened, so as to be represented by two ellipses + 

 (Diagram 20). ^ 



By I.yell and most writers the foliation here re- 

 erred to axes, is regarded as the stratification, or 

 traces of the stratification, of the metamorphic rocks 

 ot gneiss and mica-schist. The strike of the verti- 

 cal planes over Scotland seems to radiate from Do- 

 • "^g^l' ^"d is in general included between N. 25° E 

 i and N. 50° E.; but in the northern part of the Isle 

 ^ ot Lewis and the western parts of Ross and Suther- 

 I and ,t IS about N.W., or nearly perpendicular to 

 S the usual course. 



CO 



§ 6. Cleavage varies in Strata of unlike quality. 

 In a series of strata subjected to cleavage forces, 

 the result varies according to the nature of the 

 straa; perfect slaty structure being confined to 

 argillaceous, and mostly to thick-bedded argillaceous 

 deoosits. ° 



(a.) In a given section some of the strata are 

 completely traversed by cleavage, others not at all. 



In this Diagram, repre- 

 senting a section at Fig. 21. 

 Aberystwyth, 1836, the yi,-. 



I beds 5, s' are softer and 



° more argillaceous; h is 



I harder and more arenace- JlillUHiX'h 



_^ ous. The cleavage crosses 



- s and s', but is inter- 

 rupted in h. Across h, 

 however, there are gene- 

 rally found a considerable 

 number of "joints," which 

 are always more nearly 



. perpendicular to the plane 



J of the beds than the clea- 



, yage planes are. These 



I 



fart i!!f;nM^'"P^,u'^?^' "°* '"*'■'' t'le^e •" J^is s'^'-tion ; they in 

 ^3"=* coincide with his vertical dips. ^ 



t Phil. Trans. Roy. Sec, 1852, p. 445. 



