1848.] 



SHARPE ON SLATY CLEAVAGE. 



117 



separated more readily on those planes than elsewhere, and thus ad- 

 mitted them to form along them. If this be true, the laminae of talc 

 and mica must have been formed after the cleavage, and present no 

 contradiction to the views previously stated. 



Irregularities in the direction of the cleavage-planes. — There are 

 occasional irregularities in the direction of the cleavage which should 

 be studied, as they may assist our search after the general laws which 

 have regulated its arrangement. Some of these are mentioned by 

 Mr. Phillips in the Report of the British Association at Cork in 1843, 

 p. 60, and other cases have been mentioned to me verbally by that 

 gentleman. 



In one of the slate-quarries at Patterdale, a sheet of quartz about 

 an inch thick lies between two beds of slate : the cleavage does not 

 pass into the quartz, and the planes of cleavage are slightly bent out 

 of their course into a curve on each side of the quartz bed, as shown 

 in figure 4. 



Fig. 4. 

 A 



Here the lines A A and a a represent the surfaces of the different 

 beds of slate dipping to the north at an angle of 45° ; Q Q is a bed of 

 quartz interstratified in the rock. The upright lines represent the 

 cleavage-planes which dip N. 25° W. 85° : they do not enter the 

 sheet of quartz, and appear slightly deflected by it on each side for 

 the distance of about two inches. The surfaces of the beds a a are 

 not flat, but are slightly waved in parallel lines coincident wdth the 

 strike of the cleavage, a peculiarity to which we will return shortly. 



In one of the Langdale quarries there is a similar bending of the 

 cleavage against a sheet of quartz which cuts through the beddmg 

 along a joint in the rock : the circumstances are so nearly the same, 

 that it is not necessary to add another drawing. 



Such irregularities are of frequent occurrence, and their cause is 



