58 Dr. Mac Cul loch's Sketch of the 



quisitely fine grain, as also in other situations both on the western 

 and eastern shores. Considering the great extent of this rock it is but 

 rarely columnar. The most beautiful and conspicuous collection of 

 this nature is found at Little Brichel near Talisker, and ranges of 

 tolerably defined pillars are also to be seen in many places of this 

 neighbourhood occupying elevated situations in the cliffs. Columns 

 of considerable regularity are also found at Floddigary near the 

 north end of the island, and immense and continuous ranges of a 

 kind imperfectly defined occur in the vicinity of Duntulm. 



The next of the most conspicuous varieties of trap in Sky Is 

 amygdaloidal. I should perhaps in strictness rather have defined this 

 rock by its base than by Its structure or accidents, but mineraloglcal 

 language has no means of distinguishing, not only the infinite va- 

 riety, but the perpetual variation of the bases which are seen charged 

 ■with these nodules of occasional minerals. It Is sufficient to say, 

 that in hardness the base varies from basalt to almost the softness of 

 dry clay, and that the colours are black, bluish, brown, dark purple, 

 and gray of different tints, sometimes of a very pale tone. As 

 these varieties often occupy different strata, and are variously inter- 

 mixed with the solid kinds already described, the strata, when viewed 

 in the cliffs, often seem to possess a variety of composition which 

 when examined into proves fallacious. The nodules imbedded in 

 these amygdalolds are very various, few of all the substances usually 

 met with in the trap rocks being wanting In some part or other of 

 Sky. The zeolites are the most conspicuous, since all the varieties 

 but one, the Ichthyopthalmlte, Is found occupying their cavities, and 

 often in forms so large as to require a separate consideration in the 

 subsequent part of this paper. 



Calcareous spar, chlorite, steatite, quartz, chert, chalcedony, and 

 prehnite, occur in other varieties ; among which the two latter are 



