56 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



percolating from the interior of the traehytic rocks by the medium 

 of the trap-dyke ; but whether the source is from veins, or other- 

 wise, is at present unknown. 



h. Trap rocks. — The trap rocks of the islands may be regarded 

 as proved to be younger than the trachytes, from the fact just now 

 stated of the former piercing through the latter near Morton's Mill. 

 Trap dykes occur at many points of all the islands, but principally in 

 the sedimentary strata. 



c. Primary sedimentary strata. — I have already referred to a strip 

 of Primary strata lying on the north-eastern side of this island; 

 these consist, in an ascending order to the south-west, of : — 



1. A band of grey limestone, in which I detected stems of Encrinites. 



2. Purple and grey shales. 



3. Thick beds of grey limestone, seen here and there cropping out from 



under ground for the most part covered. 



The above strata, along their whole extent, are much disturbed 

 by close proximity to the traehytic rocks, their general dip being to 

 the west, and their range from north to south. Pine sections of 

 contorted strata, on a small scale, are seen here, made doubly clear 

 by the alternation of thin bands of grey and purple shales, which 

 mark distinctly the effect of side pressure. They then extend as far 

 as the ironworks mentioned above, and are succeeded by the tra- 

 chytes for about 200 yards, to be followed for about 40 yards by 

 yellow and whitish clays and thin bands of grey grit which dip 

 to the south-east at their northern end, and N. 36° "W. at their 

 southern end, at a moderate angle. The primary strata now dis- 

 appear entirely for about 350 yards farther, and come in again at 

 the Bay of San Nicolo, where white quartzose rocks are seen alter- 

 nating with the trachytes, and dipping due south at an angle of 60°. 

 Passing now across the Bay, which is covered ground, the first strata 

 seen in the cliffs, beyond the Monastery of San Nicolo, are quartzose 

 sandstones dipping S. 15° "W. at 30°, followed in an ascending order 

 by:- 



1. Claystones of yellow, purple, brown, and white colours, in part soft and 



decomposing. 



2. A bed of brown conglomerate of quartz and shale, with black bands. 



3. White quartzose rocks for about 200 yards. 



4. A bed of mottled grey and purple large-grained sandstone, with small 



imbedded pebbles of quartz. 



5. Thick beds of hard quartz rock, evidently stratified along with the other 



beds, and dipping S. 65° W. at 40°. 



6. Small bands of a soft purplish shale between beds of quartz. 



7. Thick-bedded white tabular quartz thence past a little pebbly bay to the 



extreme south-eastern point of the island, in which the stratification 

 is distinctly visible, their dip being S.45° W. at 40° at the furthest 

 point. 



Passing now along the southern side of the island, the same 

 quartzose rocks are to be seen in bold and rugged cliffs until we 

 arrive at a narrow band of fossiliferous strata forming a promontory 

 near the south-western extremity. These consist, in an ascending 

 order, of: — 



