THE SCHISTOSE ISLES. 301 



of which will hereafter be pointed out in Arran. Cer- 

 tainly, these two circumstances are by no means incom- 

 patible, and there is little doubt that our increasing 

 knowledge of the subject will hereafter confirm both 

 views, by rendering us more intimately acquainted with 

 individual instances, and consequently with the probable 

 changes which the strata of the surface, whether primary 

 or secondary, have undergone. 



In comparing the substances that form the primary 

 strata of these islands it will only be necessary to enu- 

 merate them in the most general manner, after the par- 

 ticular details already given. 



They consist of all the primary stratified rocks except 

 gneiss,^ and among them quartz rock and clay slate 

 appear predominant, micaceous schist being less abund- 

 ant. These are accompanied by a series of rocks com- 

 posed of chlorite schist, of felspar, and of hornblende, 

 in various states of mixture and alternation ; and lastly 

 by graywacke and limestone, the last being the smallest 

 in quantity. As the object of this comparison is to trace 

 the regular rocks, and as nothing can be added to what 

 was already said respecting the trap of the northern 

 islands, I shall omit further mention of it. 



It must have been remarked that the same alternations 

 of rock which occur in any one or more islands, are found 

 also nearly throughout the whole group, although in 

 different proportions. If therefore we proceed laterally, 

 by crossing the direction of the strata at right angles 

 in any number of points, we shall procure sections of 

 the alternations on those lines; assuming that they are 

 all portions of one collection of stratified substances. A 

 general correspondence will thus be found to exist 

 throughout the whole ; while no greater variations will 

 be seen to occur on the lines of bearing, than is actually 



* The portion of that rock occurring in Isla, is here passed over, as 

 it is in small quantity, and its peculiar interest was already noticed. 



