22 SHRINKAGE FISSURES OLDER ROCKS. 



piece, and cause a deep open fissure. Sometimes 

 the main joints assume curved instead of straight 

 courses. 



" The cross joints range from E. 45 K to E. 45 

 S., the most common being probably about E. 20 S. 

 They are not nearly so well marked as the main 

 joints, never being traceable for any considerable 

 distance, and seldom running even for this short 

 length in straight lines. In many places there are 

 no visible cross joints. 



"It is possible that the gentle southerly dip (or, 

 more accurately, S. 5 or 10 W.) is the cause of the 

 north and south main joints (or, more accurately, 

 those bearing N. 5 to 15 E.) being the most pro- 

 minent. The dip of the surface is the direction in 

 which water will flow ; any joint or crack in the 

 rock, therefore, lying in this direction, will be more 

 exposed to the mechanical action of the rain, than 

 one which does not lie in it. In the first case, the 

 water has its maximum velocity; in the second, it 

 will either lie stagnant, or flowing slowly, will exert 

 less mechanical force. 



"As we go westward in this district, it would 

 appear that the main joints have more of E. in their 

 bearing. In the island of Innisheer the main joints 

 are most beautifully marked, running across the 

 island, and bearing constantly E". 17 E. There are 

 also numerous main joints bearing W. of N., and 



