86 Mr Stevenson on the Geology of 



and in both the edges of fragments are translucent, and the 

 fissile property is destroyed. Near the trap-dykes the altered 

 rocks^ in some instances, contain specks of iron-pyrites. 



Such, then, are the rocks of which the island is composed ; 

 and the characteristics which render them interesting, are the 

 following. 



On the eastern side, close to the sea, at a place where stones 

 were quarried for the lighthouse works, there is a range of 

 rocks tilted up at a high angle, and of which the quarrying 

 operations have afforded a section. Among these is a bed, 

 the upper side of which appears to have been exposed to dilu- 

 vial action at some time posterior to its consolidation, as the 

 surface is uneven and rounded. Conformable with this, and 

 superior to it, is a series of very thin (^^^j inch) layers of slate- 

 band. These layers of slate have obviously been deposited 

 after the lower bed had been wasfced, as they not only rest 

 conformably with it, but have followed all the salient and re- 

 entrant angles which its water-worn surface presents. This 

 renders it probable that some considerable period had elapsed 

 between the deposit of this member of the series and the in- 

 cumbent shales. 



In the finer sorts of greywacke, I found very good specimens 

 of the graptolite or sea-pen. The ofi'-shoots or feet are visible 

 on one side only, so as to give the fossil very much the ap- 

 pearance of a saw. I have also found one specimen of the 

 double graptolite, such as was observed in Norway by Dr 

 Beck of Copenhagen. 



On the western side of the island I found a quantity of 

 steatite investing the irregular masses of the whiter sort of 

 greywacke ; nor does it traverse the rock as a vein, but seems 

 confined to one spot. Some parts of it are translucent, of a 

 greyish-yellow colour, with a shining oily lustre, and streaked 

 appearance externally ; while other parts are of a very dark 

 green, or blackish colour, and massive. 



In the coarser greywacke, formerly described as a closely 

 aggregated rock, containing a variety of fragments and peebles, 

 my friend Mr .1. T. Syme, civil-engineer, who spent some time 

 with me on the island, found part of a shell ; and I have since 

 then procured many specimens that appear to be of different 



