ISLA. GEOLOGY. 255 



in some places is found splitting into thin laminae like 

 common slate. Near Kildalton there is one very large 

 vein which must be ranked with these, although, in struc- 

 ture, it could not be distinguished from a syenitic granite; 

 not the only instance in which the mineralogical bounda- 

 ries of these and of the trap rocks are difficult to define. 



Quartz veins also occur in this island ; a feature too 

 common to be worthy of notice were not some of them 

 attended by a remarkable circumstance, namely, an inter- 

 mixture of brilliant oxidulous iron in thin reticulating 

 lamina and in great quantity. A very conspicuous ex- 

 ample of this nature occurs near Portaskeg. 



There is yet a circumstance in Isla connected with 

 the consideration of veins, which it is necessary to notice, 

 because it seems to have led to erroneous conclusions. 

 This is the supposed existence of veins of clay slate, an 

 appearance equally common in Scarba and Jura, and 

 indeed not very rare. These supposed veins are always 

 contorted and irregular, while they are also interrupted 

 by veins and fragments of quartz.* They are either 

 bounded by quartz rock, or by other materials ; in these 

 islands the walls are of the former substance. When 

 they are carefully examined and their continuations are 

 traced, it will be found that they are portions of beds 

 which have undergone the contortion and elongation 

 so frequent in micaceous schist and gneiss, and are 



*thus drawn out into forms slightly resembling real 

 veins. Some of the contortions of gneiss which occur 

 in the Long island are precisely of the same character, 

 and derived from similar causes. The deception, if 

 such it may be called, produced by these contorted and 

 elongated portions of stratified rock, is occasionally such, 

 that it requires some attention to avoid being misled 

 by them. In .describing the hill of Kinnoul, on a former 

 occasion, some remarkable examples of these appearances 



Plate XX [ r. 



