CANNA. DESTRUCTION OF ROCKS. 463 



too near with the land. I may add that the same circum- 

 stance takes place in the island of Muck and in the Isle 

 of Man. In these latter examples limestone is the pre- 

 vailing substance ; but it is worthy of notice that even the 

 very soft shale found in Pabba and on the Broadford 

 shore, seems to lose its decomposing property as soon as 

 it has wasted to that level where it becomes exposed to 

 the diurnal action of the sea water. 



It may be perceived by the instances thus adduced, that 

 the most conspicuous examples are found in the trap 

 rocks and in the secondary strata, but the effect does 

 not appear limited to these alone. To the same cause 

 are doubtless owing the projecting rocks which are almost 

 invariably found to skirt rocky shores of whatever sub- 

 stance these may be composed ; the further ruin of the 

 cliffs appearing to meet with a check wherever the sea 

 comes into contact with them. The difference in the 

 apparent effects in the case of these rocks, seems partly 

 to arise from the different mode in which they are 

 disposed, in consequence of which the waste cannot take 

 place in angles so nearly vertical, nor the effect arising 

 from contrast be so strongly perceived!; and partly, it is 

 to be presumed, from their greater durability and more 

 tedious destruction. 



It is perhaps not difficult to assign the causes of this 

 phenomenon. The ordinary causes of decomposition are 

 too well known to require enumeration : and one of these 

 appears equally to act in both cases ; namely, the change 

 produced in the iron of the compounds by the access of 

 air and water. But from the other and next most obvious 

 cause of decomposition, namely, the action of frost, it is 

 evident that the rocks are in one situation excluded, while 

 they are exposed to it in the other. To this we are per- 

 haps to assign the whole difference above mentioned ; and 

 we are thus led in these colder regions, to ascribe a much 

 greater power in changing the face of the earth to this 

 agent, than to any chemical actions 1 ; the operations of 



