166 % LUNGA. GEOLOGY. 



leading line of the strata, and because its common 

 boundary with the quartz rock is not parallel to that 

 line. Although this rock is here predominant, it does 

 not maintain exclusive possession, since it alternates 

 with micaceous schist. Both these substances pre- 

 sent very decided characters ; the latter being brilliant 

 and composed of grey mica interlaminated with quartz ; 

 the argillaceous schist differing in no respect from that 

 of the Slate isles, and being capable of affording slates 

 similar to those of Eysdill. 



Although the belt of argillaceous schist is thus described 

 as if it were separated from the quartz rock, there is 

 no defined boundary between them ; various alternations 

 taking place before the latter is established to the ex- 

 clusion of the former. It cannot indeed be said to 

 be ever thoroughly excluded ; since beds of the simplest 

 argillaceous schist are found alternating with quartz rock 

 of a perfect character, even to the western side of the 

 island, and nearly down to the bottom of the lowest 

 accessible strata. Nor, on comparing the structure of 

 Lunga with that of the remainder of this chain of islands, 

 can any point be fixed on in the quartz rock, however 

 thick its mass, below which beds of that schist are not 

 found. 



The beds of quartz rock which, under this exception, 

 occupy the western side of Lunga, exhibit elevations cor- 

 responding to those of the schist. As the passage of 

 the sound formerly described, affords an opportunity of 

 seeing their transverse sections in great perfection, the 

 incurvations which cause their elevated planes to assume 

 so many different angles when examined on the lines 

 of bearing, are brought fully into view ; affording a very 

 instructive explanation of the circumstances which, in 

 other situations, produce the varying inclination of similar 

 (strata. The beds of quartz rock differ in thickness from 

 one foot to forty or more; and the causes of separation 

 among them will generally be found to consist in the 



