| Abstract/Notes | In a paper read on February 23rd, 1876, before the Geological Society of London, I endeavoured to prove that the peculiar laminated or fissile structure, common in certain igneous rocks, was due to contraction, like the associated ordinary joint-structures. In the discussion which followed, an objection was made to my having quoted certain phonolites from Auvergne, in support of my theory, because it was well known that the fissile structure in the typical phonolites of Hungary was due to change in mineral composition, and so the result of a banded structure in the rock. That this was not the case with those Auvergne phonolites, I had already convinced myself by examination, both in the field and with the microscope, and I had nowhere asserted that the only way in which rocks could become fissile was by the particular cause which I had advanced; for, although I only knew the Hungary rocks from hand-specimens, I was aware of a somewhat similar structure in certain British rocks. I thought I remembered also one or two cases in these where the fissile and the banded structure were co-existent and independent, but as the criticism came from a quarter not to be neglected, I determined to re-examine these cases. This I have done on occasion of short visits to Arran in the past and present year, during which I have obtained materials which will, I think, further illustrate this, and throw some light on other rock structures. The points, then, upon which I shall touch in the present paper are the relation (I.) of fissile and banded structures; (II.) of banded, spherulitic, and perlitic structures. |
|---|
