398 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 com- 

 mon 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 de- 

 fined 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 investi- 

 gation, 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 corneene and trap have been applied, names 

 which will probably be given to this rock b)'' 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 con- 

 sidered 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 ob- 

 served upon the planes of contact. I did not find any specimen of 

 wavellite in this variety ; but I cannot pass on without remarking 



