SCARBA. GEOLOGY. 



both to alternate with, and pass into, micaceous schist ; and 

 thus a rock is often formed, partaking in such a manner 

 of both these, that it may with equal propriety be ranked 

 with either. As the geological relations of these two 

 substances are the same, it is not necessary to distin- 

 guish them in describing their alternations with the clay 

 slate. These may be seen all along the eastern side of 

 the island ; the several beds succeeding each other with- 

 out any determined order, and the clay slate, as might 

 be expected from the preceding remarks, sometimes alter- 

 nating with the quartz rock, at others with the micaceous 

 schist. If there is in either case a gradation at the line of 

 junction, it is very slight. It is evident that this clay 

 slate is the same rock that constitutes the Slate islands, 

 formerly described ; since, as I have already shown, it 

 forms a part of the same series of beds, however geogra- 

 phically interrupted. It is perfectly soft and fine grained, 

 of a dark blue colour, silky and undulated on the surface, 

 and contains cubical crystals of pyrites. As its alternation 

 with the first named set of rocks, is to be seen on the eastern 

 side of the island, so the regular interchange of the two 

 is visible in the transverse section of the whole on the 

 southern shore. The undulation of the mass of strata 

 which here occurs, is very remarkable, and it is repre- 

 sented in a general way in the accompanying section.* 

 It is impossible that any error can have intruded itself 

 into these observations, since the whole is as palpable 

 as if it were presented in a model. 



Trap veins of considerable magnitude are found here, 

 and they are probably the causes of those disturbances 

 visible in the rocks, which are independent of the general 

 undulation. These disturbances consist in a confused inter- 

 mixture of the quartz rock with the clay slate ; the even- 

 ness and order of both being interrupted, and fragments 

 of the one being irregularly interspersed among the other. 



Plate XXIII. fig. 1. 



