344 ARRAN. GEOLOGY. 



tracts of peat and the luxuriant growth of heath and 

 other moor plants, that the rocks are seldom accessible. 

 In general it is not difficult to determine the extent 

 of many kinds of rock, from the mere comparison of 

 remote and occasionally protruding masses. With regard 

 to the secondary strata, or the stratified rocks in general, 

 that determination is commonly easy ; from the expe- 

 rienced continuity and regularity of these substances 

 in other situations where they are accessible. But in 

 the rocks which are not stratified, and more particularly 

 in that class which constitutes this part of Arran, 

 no such rules can be relied on ; as they are notoriously 

 subject to interruptions , and to be intermingled with 

 the secondary and stratified rocks with which they are 

 associated. Where more than one species of this 

 family occurs, the determination becomes still more 

 intricate, and seems indeed to defy all powers of con- 

 jecture or comparison. This is the case with the tract 

 in question, where many different rocks of this nature are 

 found ; all possessing the common character of being con* 

 nected with the stratified substances, in a position some- 

 times inferior, in others superior to them. In such circum- 

 stances, even a map is unavailing for the explanation; 

 since points alone can be marked, and even these can 

 scarcely pretend to accuracy of position unless a geo- 

 metrical survey were made for that especial end ; a 

 labour which would not be repaid by any corresponding 

 advantages, either in geological science or topographic 

 mineralogy. I must refer the reader to the accom- 

 panying sketch of a map ; and need only here add, 

 that the rocks which constitute all those parts of this 

 division which are not sandstone, are varieties of trap, 

 syenite, porphyry, and other rocks of this family, the 

 peculiarities of which must be considered in the mine- 

 ralogical details hereafter to follow. 



In describing the rocks of this island according to 

 their more minute geological details, I shall take 



