Smeeth — On the Dolomite of Howth. 275 



dency for them to split in that way instead of indiscriminately. 

 The most probable cause of such a tendency is pressure ; and on 

 examining the surrounding district it is seen that the beds of lime- 

 stone (on the edge of which the dolomite occurs) have been bent 

 into a kind of basin : one effect of this would have been to com- 

 press the portions round the edge tangentially, thereby setting up 

 a tendency to cleave in directions parallel to the dip, i.e. approxi- 

 mately in the same direction as that in which the parallel cracks 

 are found to extend. 



EXPLANATION OP PLATE III. 



Fig. 1. — Fragment of Dolomite from Sutton, Howth, County Dublin, 

 showing the parallel cavities. Slightly enlarged. 



Fig. 2. — Part of a section of the above, magnified to show the structure 

 about one of the cavities. 



(d) Dolomite Crystals. 



(/) Lining of ferruginous matter. 



(c) Calcite grains filling up the cavity. 



