176 Dr. Mac Culloch's Supplement to the 



strewed on the shore proves that this substance also exists some- 

 where in the cliffs ; doubtless under the same circumstances which 

 I formerly described at Duntulm. I must add that the specimens 

 sometimes contain shells, and that, resembling basalt in appearance 

 and texture, they confirm the truth of those suspicions respecting 

 the asserted existence of organic substances in that rock, which it 

 is here sufficient to have mentioned. 



Together with these detached blocks of siliceous schist are found 

 similar fragments of a cherty substance, extremely hard and brittle, 

 and breaking into acute conchoidal fragments, but possessing an 

 earthy aspect. Its colours vary from greyish white to dark smoke- 

 grey, and I may add that its degrees of induration are also various. 

 Occasionally, portions of the siliceous schist are attached to it, the 

 separation being marked by well defined planes, and, from the 

 contrast of colour, very conspicuous. If there were any doubt that 

 this chert was originally a portion of the lias indurated by the same 

 process that has converted the shale into siliceous schist, it would 

 be removed by the fact that on the western shore of this district 

 the two substances are found in situ, associated in the same manner 

 and in various states of transition from common lias and shale to 

 chert and siliceous schist. 



The last portions of limestone to be seen on this shore occur at 

 the island of Fladda, occupying a very low position, and at length 

 disappearing gradually below the trap, which beyond this point 

 forms the whole coast as far as Duntulm, constituting also the 

 islands of Trodda and Fladdahuna, as well as the various picturesque 

 rocks which are scattered to the north of the point of Hunish. 

 This bed of limestone abounds in organic remains, but so condensed 

 together, and so broken, as to present no specimens capable of being 

 ascertained : they resemble fragments of some sort of cockle and of 

 anomiae, or perhaps ostrese. 



