MINERALOGY OF THE LIZARD DISTRICT. 181 



opinion that "the gabbros of Coverack closely resemble the Tertiary 

 gabbros of the Western Islands of Scotland," is a very important 

 feature in working out the volcanic origin of the Lizard rocks. 



Prof. Bonney has admitted that he examined the district at an 

 unfortunate epoch, and had consequently fallen into one error, viz., 

 among the banded series occurred porphyritic felspar, which he now 

 knows is usually indicative of igneous origin, and had overlooked 

 several masses which he would now be inclined to regard as igneous.* 



The banded and foliated appearance of the hornblendic schists of 

 this locality I attribute to the varying composition of the materials 

 supplied to the crater, not an uncommon thing in volcanic ejectments. 

 A similar banded appearance is recorded of the lapilli covering 

 Pompeii, which may be attributed in that case to the material of the 

 highest specific gravity — in its passage through the atmosphere — 

 first reaching its resting place. The same is also recorded of the 

 obsidians of Hungary and the Lipari Islands, and I find, on 

 examination, the felspathic tufa from the Kathkin hills, near 

 Glasgow, also shows a banded appearance, resembling that of the 

 Porthallow rocks. 



The outcrop of the Manacle sheet, with the great variations of 

 its predominant minerals in so short a distance, impels me to the 

 conclusion that this was the source from which the banded structure 

 lying between Porthoustock and Porthallow flowed, f and were the 

 same mass in motion to-day, no other result could be obtained. 



In proof of its volcanic origin in contra distinction to plutonic 

 production or contact metamorphism, I submit the following : 



i. The lower beds or sheets contain traces of matter that has 

 passed through many changes since its first selective process, at 

 high temperature, into rock-forming minerals, and sheet by sheet as 

 as we ascend newer and fresher fire-formed minerals appear, until 

 we arrive at a point where no perceptible change through age is 

 traceable ; all are clear and bright as of recent finish. 



2. Had such minerals been of plutonic origin the most perfect 

 crystals would be found in the lowest beds. 



*Quar. Jour. Geo. Soc, Lon., 1889, page 543. 



fSee Map in No. 35 Journal, R.I.C., Oct., 1889, and Microscopic section. 



