Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/408

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Dr. Mac Culloch on the Geology of

trace no difference between the amorphous portion and the columnar mass which forms the upper part of the island. This very common fact should serve to convince us how little information we can obtain of the true and fundamental structure of rocks, from their natural fracture, while in a state of integrity.

The bed of trap which I have now described, is followed by a bed of schistus, which may be called a siliceous schistus, a term likewise under which many substances have been very improperly confounded, although the characters of this rock are sufficiently defined to render such carelessness of description inexcusable.

But it is not only in mineralogy that the desire of accounting for that of which the cause is unknown, and of describing that which is not understood, predominates over those severer habits of investigation, which can alone render scientific description accurate.

It is too hopeless a task to attempt to develope the confusion of rocks to which the terms cornèene and trap have been applied, names which will probably be given to this rock by those who follow the French nomenclature. I believe I go along with Mr. Jameson, in calling it flinty slate, or siliceous schist, and it will perhaps be considered a variety of that which is called Lydian stone. It breaks naturally into prismatic and rhomboidal fragments, but its forcible fracture is flat, approaching to the conchoidal, with a small degree of lustre, and without grain. It is exceedingly brittle, and cuts the hands like glass. Its gives fire readily with steel, and the file scarcely touches it. Its colour is a dark lead grey.

Natural joints, similar to those I noticed in the botryoidal schistus, (if I may venture thus to distinguish the rock described above) occur also in this schistus, and circular stains may be observed upon the planes of contact. I did not find any specimen of wavellite in this variety; but I cannot pass on without remarking