20 ION A. GEOLOGY. 



in every possible complication of structure and position, 

 are continued to the extremity of the island. 



One peculiarity attends all the rocks of lona, namely, 

 the great abundance of a substance hitherto considered as 

 compact epidote, which is every where found in them. It 

 is particularly prevalent in that bay, situated towards 

 the west, known by the name of Port na Curachan, 

 where St. Columba is said to have first landed, and 

 which is marked by large conical heaps of pebbles, the 

 penitentiary labours, as tradition says, of pilgrims to 

 his shrine*. It here forms either large lumps or la- 

 minae imbedded in the gneiss, and in some situations 

 enters into it as a constituent part. In other places, 

 it is found mixed with hornblende in various ways, or 

 else serving for a base in which crystals of hornblende 

 are imbedded; forming a rock which has been mis- 

 taken for serpentine. 



As this substance does not appear to be veiy common, 

 and seems to have in a great measure escaped attention, 

 a description of its predominant characters will not be 

 superfluous. 



It is generally pale green, at times approaching to 

 yellow. The fracture is intermediate between the con- 

 choidal and flat splintery, and it is exceedingly difficult 

 to break, the fragments being slightly translucent on the 

 edges. It does not easily yield to the file, and scratches 

 quartz, while in return it is scratched by that substance. 

 Its specific gravity is the same as that of quartz, but 

 it has not been analyzed. From a comparison of the 

 characters of numerous specimens selected from different 

 places, I am inclined to consider it as a variety of 

 compact felspar, and it will indeed be found to pass 

 occasionally into the more common varieties of that 

 substance. It forms a conspicuous portion of a gneiss 

 abounding on the west coast of Ross-shire. 



* In this spot is found that beautiful and rare Conferva, the atni of 

 Dillwyn; not in streams, but in standing pools. 



