Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/66

 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 mineralogical language has no means of distinguishing, not only the infinite variety, 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 intermixed 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 amygdaloids 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 ichthyopthalmite, 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