IONA. GEOLOGY. 19 



semblance of a direct continuity. Those hitherto de- 

 scribed are either argillaceous schist, or contain that 

 substance as a leading ingredient; but at this point 

 the siliceous matter becomes predominant, or, at least, 

 very conspicuous. These rocks have frequently the ex- 

 ternal aspect of granite, and have been mistaken for 

 granite veins, which on a distant view they resemble. 

 The granitic ingredient is a compact mixture of felspar 

 and quartz, which is either disposed in laminae in the 

 schist, or is mixed with it in an irregular manner. Thus 

 the rock resembles a gneiss, though it consists, in 

 strictness, of a laminated mixture of granite and clay 

 slate. It alternates, as far as alternation can be pro- 

 nounced of rocks so irregular, with the hard argillaceous 

 schist. 



This rock is therefore either unprovided with a name, 

 or else it must be considered as a variety of gneiss. 

 If so, both the geological position and the definition 

 of gneiss require to be corrected, since it alternates here 

 with clay slate, and also contains it as an ingredient. 

 Both these circumstances have, in fact, been prematurely 

 limited, and the connexions and definition of this rock, 

 like that of many others, established on too narrow a 

 foundation. 



The structure of the remaining part of the island con- 

 firms this view of the nature of the rock above described. 

 It may be stated in general terms, as there appears nothing 

 further in its various aspects requiring a minute detail. By 

 degrees the anomalous gneiss gives way to one of a more 

 ordinary structure, consisting of quartz, felspar, and horn- 

 blende, disposed in a regularly laminated form, and 

 containing beds both of common hornblende slate, and 

 unlaminated granite ; mixtures almost always found to 

 accompany each other in this country in those gneiss 

 rocks of which hornblende forms an ingredient. Still 

 further towards the north end of the island, gneiss con- 

 taining mica is to be seen, and of this, several varieties, 



