444 SHIANT ISLES. MINERALS. 



and elsewhere, where analogous substances are found in 

 the vicinity of trap. Such variations have been used as 

 an argument against the igneous origin of trap rocks ; but 

 it is scarcely necessary to remark, that among a series 

 of effects attributed to an assumed cause, an exception 

 does not invalidate its efficacy, unless we were in posses- 

 sion of all the collateral circumstances which might have 

 influenced its action. 



A few mineral substances are to be seen in these islands, 

 not necessarily attached to the rocks of which they are 

 formed. Among these are considerable deposits, in the 

 intervals of some of the columns, of that calcareous matter 

 known to the Germans by the name of rock milk; an 

 occurrence not very unusual among trap rocks. I observed 

 on the beach a rolled mass of pink opalescent quartz ;. but 

 this must have been brought by the waves from some 

 distant place, as quartz of this nature is not an inmate of 

 the rocks of the trap family. 



The last and most important mineral which I found was 

 one of considerable rarity, wavellite, hitherto discovered in 

 but few places. It occupies rifts in the siliceous schist, 

 but occurs very sparingly. I mentioned it long ago in the 

 Transactions of the Geological Society, in a slight and 

 imperfect notice, the result of a first and very hurried visit 

 to these islands. 



