THE ISLE OF EIGG. 147 



in position relative to each other or to the .stratum in 

 which they occur, they preserve no regularity, but are 

 either dispersedly scattered through a stratum at con- 

 siderable distances from each other, or are in imme- 

 diate contact, forming then concretionary layers arranged 

 either parallel to the direction of the strata, or inclined at 

 various angles to their planes. An examination of the inte- 

 rior of the concretions affords no determinate arrangement, 

 and they only differ from the sandstone in which they are 

 included by the circumstance of being more compact. 

 Though, in external aspect, the globular sandstone of Eigg 

 may appear similar to that globular concretionary struc- 

 ture observable in the trap-rocks, still there is much to 

 render it probable, that these appearances have been pro- 

 duced by the action of completely distinct causes. When 

 a globular trap-rock is exposed to the action of the wea- 

 ther, its disintegration is produced by the exfoliating of 

 concentric layers. This character is universal, but is never 

 to be seen in the sandstone of Eigg ; and further, the balls 

 of trap are not irregularly scattered through the trap ba- 

 sis. In a trap-rock the globular structure is generally ex- 

 hibited over many yards, or throughout the whole mass ; 

 while, in the case of the sandstone, one concretion only may 

 be found in a stratum. From the circumstance of the la- 

 minae of the sandstone, in some instances, not terminating 

 at the surface of the concretion, but, on the contrary, ex- 

 tending through them, it is evident that the cause which 

 produced this arrangement operated after the deposition of 

 the strata. There was a tendency to effect a symmetrical 

 appearance ; but what generated this we are ignorant of, 

 and though the agents may have been of a similar nature, 

 yet they are as obscure as those which have awarded one 



